Document Labkit::RateLimit in repo README and add module README

Summary

The labkit-ruby README does not currently document the Labkit::RateLimit functionality, even though the module is shipped, in use, and being rolled out across GitLab Rails via Gitlab::ApplicationRateLimiter::LabkitAdapter. There is also no README inside lib/labkit/rate_limit/ to explain the rule/identifier/result model and the Redis-backed evaluator, which would help onboarding for consumers and maintainers.

This issue tracks adding both pieces of documentation.

Scope

  1. Add a Labkit::RateLimit entry to the Functionality list in the root README.md, with a pointer to the module README (same pattern as Labkit::Metrics, Labkit::UserExperienceSli, Labkit::RSpec).
  2. Create lib/labkit/rate_limit/README.md covering:
    • What the module does and how it differs from a divmod-window limiter
    • Configuration (global Labkit::RateLimit.configure, Redis connection pool requirement)
    • Constructing a Limiter and the check / peek API
    • Identifier and the characteristic / match model
    • Rule semantics: limit, period, action (:block / :log / :allow), match, characteristics, and the first-match-wins evaluation order (with the special non-terminating behavior of :log)
    • Result API including to_response_headers
    • Emitted Prometheus metrics (names and labels)
    • Fail-open behavior on Redis error

Acceptance criteria

  • Root README.md lists Labkit::RateLimit with a link to the module README
  • lib/labkit/rate_limit/README.md exists and covers the items above
  • MR linked to this issue and to the parent epic

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