docs(rate_limit): add module README and link from root README

1. `Labkit::FIPS` for checking for FIPS mode and using FIPS-compliant algorithms.

1. `Labkit::Logging` for sanitizing log messages.

1. `Labkit::Metrics` for metrics. More on the [README](./lib/labkit/metrics/README.md).

1. `Labkit::RateLimit` for rules-based, Redis-backed rate limiting. More on the [README](./lib/labkit/rate_limit/README.md).

1. `Labkit::RSpec` for RSpec matchers to test Labkit functionality (requires selective loading). More on the [README](./lib/labkit/rspec/README.md).

1. `Labkit::Tracing` for handling and propagating distributed traces.

# Labkit::RateLimit

`Labkit::RateLimit` is a rules-based rate limiter backed by Redis counters. It

maintains a fixed-window counter per `(call-site, rule, characteristics)` tuple

and decides whether each request is within the configured limit.

The module is intentionally small: a `Limiter` is configured at boot with an

ordered list of `Rule`s, and every request calls `Limiter#check(identifier)` to

get back a `Result` describing what the caller should do.

## Configuration

`Labkit::RateLimit.configure` sets a global Redis connection pool and logger

that are reused across all `Limiter` instances unless a per-Limiter override is

supplied:

```ruby

Labkit::RateLimit.configure do |c|

  c.redis  = ConnectionPool.new(size: 5) { Redis.new(url: ENV["REDIS_URL"]) }

  c.logger = Labkit::Logging::JsonLogger.new($stdout)

end

```

The `redis` value must respond to `.with { |conn| ... }` and yield a connection

that supports `incr`, `ttl`, `get`, `expire`, and `pipelined`. A

`ConnectionPool` of `Redis` clients is the typical choice.

The logger is used only for warnings (invalid rule names, fail-open errors,

duplicate rule names in production). It defaults to a JSON logger writing to

`$stdout`.

## Defining a Limiter

A `Limiter` is the unit of configuration for one call site (e.g. "rack

requests", "graphql mutations", "ai actions"). Construct it once and reuse it:

```ruby

RACK_LIMITER = Labkit::RateLimit::Limiter.new(

  name: "rack_request",

  rules: [

    Labkit::RateLimit::Rule.new(

      name: "api_user",

      limit: 600,

      period: 60,

      characteristics: [:user],

      match: { endpoint: { re: '\A/api/' } }

    ),

    Labkit::RateLimit::Rule.new(

      name: "unauthenticated",

      limit: 60,

      period: 60,

      characteristics: [:ip],

      match: { user: nil }

    )

  ]

)

```

- `name` must match `/\A[a-z0-9_]+\z/`. It is used as the first segment of

  every Redis counter key for this limiter, so renaming a `Limiter` abandons

  any in-flight counters.

- `rules` is an ordered array of `Rule` objects. The first rule whose `match`

  hash is satisfied wins (with the exception of `:log` rules — see [Actions](#actions)).

- `redis` and `logger` are optional; they fall back to the global

  `Labkit::RateLimit.config` values.

A `Labkit::RateLimit.check(name:, identifier:, rules:, ...)` convenience method

exists for one-off cases that cannot cache a `Limiter` instance, but it

allocates a fresh `Limiter` on every call and is not the recommended path.

## Checking a request

`Limiter#check(identifier)` increments the counter for the matched rule and

returns a `Result`. `identifier` is either an `Identifier` or a plain `Hash`

of caller attributes:

```ruby

result = RACK_LIMITER.check(

  user:     current_user&.id,

  ip:       request.ip,

  endpoint: request.path

)

if result.exceeded? && result.action == :block

  response.headers.merge!(result.to_response_headers)

  render plain: "Too Many Requests", status: 429

  return

end

```

The `endpoint` key is treated specially: the query string is stripped at

`Identifier` construction time so URLs that vary only by query parameter share

the same counter.

### Peeking without incrementing

`Limiter#peek(identifier)` returns the same `Result` shape but does not

mutate Redis. It is useful when one code path should account for the request

(`check`) and another should gate a side-effect on whether the caller is

already over-limit. `peek` skips `:log` rules — their state is unobservable

without incrementing.

## Identifier

`Identifier` is a small value object wrapping a hash of caller attributes.

You can pass a `Hash` to `check`/`peek` and `Limiter` will wrap it for you, or

construct one explicitly:

```ruby

id = Labkit::RateLimit::Identifier.new(

  user:     42,

  ip:       "1.2.3.4",

  endpoint: "/api/v4/projects/1?per_page=20"  # becomes "/api/v4/projects/1"

)

```

Keys can be symbols or strings — they are normalised to symbols on the way

in.

## Rule

A `Rule` is a `Data.define` value object with the following fields:

| field             | meaning                                                                                                                                                              |

|-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|

| `name`            | Stable identifier used in Redis keys and metric labels. Must match `/\A[a-z0-9_]+\z/`, max 64 chars. Renaming a rule abandons its in-flight counters.                |

| `match`           | Hash of identifier key/value predicates that must **all** be satisfied for the rule to apply. Empty hash matches anything. See [Matchers](#matchers).                |

| `limit`           | Integer request threshold per `period`. May be a callable resolved on every check.                                                                                   |

| `period`          | Window length in seconds. May be a callable resolved on every check.                                                                                                 |

| `action`          | What the result reports when the limit is exceeded. One of `:block`, `:log`, `:allow`. Default `:block`. See [Actions](#actions).                                    |

| `characteristics` | Array of identifier keys whose values are folded into the Redis counter key. Each unique combination gets its own counter.                                           |

Making `limit` or `period` callable is the supported pattern for

runtime-tunable thresholds (e.g. feature flags or database-backed settings):

```ruby

Labkit::RateLimit::Rule.new(

  name: "api_user",

  limit:  -> { Settings.rate_limit_api_user_per_minute },

  period: 60,

  characteristics: [:user]

)

```

### Matchers

A `match` hash gates whether a rule applies. Each value is normalised through

`Matcher.build`:

| input shape       | matcher  | example                                                |

|-------------------|----------|--------------------------------------------------------|

| plain value       | `eq`     | `match: { user: nil }`, `match: { method: "POST" }`    |

| `Regexp`          | `re`     | `match: { endpoint: %r{\A/api/} }`                     |

| `{ eq: <value> }` | `eq`     | `match: { method: { eq: "POST" } }` (YAML-friendly)    |

| `{ re: <source> }`| `re`     | `match: { endpoint: { re: '\A/api/' } }` (YAML-friendly) |

`re` coerces the identifier value via `#to_s` before matching, so it can be

used against non-String values (e.g. matching a 503 status against `{ re: '^5' }`).

Glob and prefix matchers are intentionally out of scope.

### Actions

The rule's `action` controls how the `Result` reports an over-limit hit. The

counter is always incremented when a rule matches, regardless of `action`:

- `:block` — when exceeded, `Result#action` is `:block`. Caller should reject

  the request (e.g. with HTTP 429). When under the limit, action is `:allow`.

- `:log` — **non-terminating**. The rule counts the request and records

  metrics, but evaluation continues to the next rule. This is the mechanism

  for shadow rules during rollout: stack a `:log` rule and a `:block` rule

  together and the `:log` rule cannot disable the `:block` rule. Note that a

  pure `:log`-only check still emits one `rule="unmatched"` metric entry

  because no terminating rule fired.

- `:allow` — when exceeded, `Result#action` is `:allow` (rather than

  `:block`). Useful for "always allow this caller even if they're over the

  limit" cases while still observing them via metrics. Evaluation terminates

  on the first match.

### Redis keys

Each matched check writes a key shaped:

```

labkit:rl:<limiter_name>:<rule_name>:<char>:<value>[:<char>:<value>...]

```

Characteristic values longer than 200 bytes are replaced with a SHA-256

hexdigest to bound key length. Missing or empty characteristic values are

encoded as `_unknown_`. The TTL is set on the first write of each window

(`count == 1`) and is not extended on subsequent INCRs, so the window is a

true fixed window starting at the first request, not a sliding window.

## Result

`Result` carries the decision back to the caller:

```ruby

result.matched?           # => true if some rule matched

result.exceeded?          # => true if the matched rule's counter > limit

result.action             # => :block | :log | :allow

result.rule               # => the matched Rule, or nil

result.error?             # => true if Redis failed (see Fail-open)

result.info               # => Result::Info or nil

result.to_response_headers

# => { "RateLimit-Limit" => "...", "RateLimit-Remaining" => "...", "RateLimit-Reset" => "<unix-ts>" }

```

`Result::Info` holds the per-window counter snapshot:

| field             | meaning                                                          |

|-------------------|------------------------------------------------------------------|

| `resolved_limit`  | The evaluated `Integer` limit for this check.                    |

| `resolved_period` | The evaluated `Integer` period in seconds for this check.        |

| `count`           | Raw INCR value; useful for utilization-ratio metrics.            |

| `remaining`       | `[resolved_limit - count, 0].max`.                               |

| `reset_at`        | Best-effort UTC `Time` when the window resets (advisory only).   |

`to_response_headers` returns `{}` for an unmatched or error result, so it is

safe to merge unconditionally.

## Fail-open

The evaluator wraps `check` and `peek` in a broad rescue. Any `StandardError`

(Redis connection failure, timeout, OOM in user-supplied callables, …) is

logged at WARN with `message: "rate_limit_error"` and returned as a

`Result(matched: false, error: true, action: :allow)`. The

`gitlab_labkit_rate_limiter_errors_total` counter is incremented. The caller

should treat the request as allowed.

## Metrics

`Labkit::RateLimit::Metrics` emits the following Prometheus metrics through

`Labkit::Metrics::Client`:

| metric                                          | type    | labels                              | meaning                                                              |

|-------------------------------------------------|---------|-------------------------------------|----------------------------------------------------------------------|

| `gitlab_labkit_rate_limiter_calls_total`        | counter | `rate_limiter`, `rule`, `action`    | One increment per terminating decision; also incremented per matched `:log` rule. `action` is one of `"allow"`, `"block"`, `"log"`. `rule="unmatched", action="allow"` when no rule terminated. |

| `gitlab_labkit_rate_limiter_errors_total`       | counter | `rate_limiter`                      | Fail-open events (any `StandardError` in the labkit path).            |

| `gitlab_labkit_rate_limiter_limit`              | gauge   | `rate_limiter`, `rule`              | Resolved limit at the last check (useful when `limit:` is callable). |

| `gitlab_labkit_rate_limiter_period_seconds`     | gauge   | `rate_limiter`, `rule`              | Resolved period at the last check.                                   |

Because `:log` rules do not terminate, a single `check` call can emit

**multiple** `calls_total` increments: one per `:log` rule that matched, plus

one for the terminating decision (or `rule="unmatched"` if no terminating

rule fired).

## Dev/test vs production guards

`Limiter.new` and `Rule.new` validate names and configuration. In

`Labkit.dev_or_test?` mode (`RAILS_ENV` set to `development` or `test`), they

raise `ArgumentError` on:

- invalid limiter or rule names

- duplicate rule names within a single `Limiter`

- unknown `action` values

- rule names longer than 64 characters

In production, the same conditions are downgraded: invalid names are

sanitised (and the original/sanitised pair is logged), duplicate rule names

are dropped (first occurrence wins), and a warning is logged. This keeps a

misconfiguration from taking the application down at boot.

    # period          - window in seconds; may be a callable (resolved per check)

    # action          - :block (enforce), :log (count and log only, do not block,

    #                   evaluation continues to subsequent rules), or :allow

    #                   (bypass: short-circuit evaluation with no Redis writes)

    #                   (count but always permit; terminates evaluation on match

    #                   regardless of whether the limit was exceeded)

    # characteristics - identifier keys used to build the compound Redis counter key

    #

    # +name+ must be a lowercase alphanumeric-and-underscore string of at most 64