Add Prometheus instrumentation to Rails-side Orbit components

What does this MR do and why?

Adds Prometheus histograms and counters to the Rails-side Orbit components (GrpcClient, JwtAuth, AuthorizationContext, RedactionService). The GKG/Rust side has query pipeline timing and ClickHouse profiling, but the Rails hot path had no metrics, so we couldn't tell where latency was coming from.

This is the instrumentation MR in the redaction scalability series. See orbit/knowledge-graph#348 for the full plan.

New metrics module ee/lib/gitlab/metrics/knowledge_graph/request.rb, following the existing TraversalIds pattern:

Metric	Component	Description
gitlab_knowledge_graph_grpc_duration_seconds	GrpcClient	Duration per RPC method (method/status labels)
gitlab_knowledge_graph_grpc_errors_total	GrpcClient	Errors by method and gRPC status code
gitlab_knowledge_graph_jwt_build_duration_seconds	JwtAuth	End-to-end generate_token duration (includes auth context)
gitlab_knowledge_graph_auth_context_duration_seconds	AuthorizationContext	Traversal ID computation on cache miss
gitlab_knowledge_graph_redaction_duration_seconds	RedactionService	Duration of execute (resource loading + ability checks)
gitlab_knowledge_graph_redaction_batch_size	RedactionService	Resources evaluated per batch
gitlab_knowledge_graph_redaction_filtered_count	RedactionService	Resources denied per batch

Design decisions

• measure_grpc uses ensure so duration is always recorded, even on exception (status label becomes "error")
• RedactionService accepts metrics_observer: — a callable dependency injected by the caller, matching the existing logger: pattern. This keeps the FOSS service generic and decouples it from EE metrics constants. The Workhorse internal redaction endpoint can inject its own observer.
• redaction_duration_seconds measures RedactionService.execute — timed inside the service itself (resource loading + ability checks), not the gRPC streaming exchange. This survives the Workhorse migration (ADR 008) since RedactionService is called from both Rails and the Workhorse internal redaction endpoint.
• FOSS compatibility — metrics calls live in EE code only; the FOSS RedactionService uses the injected observer without referencing any EE constants.

How to set up and validate locally

git checkout arivera/rails-metrics-capture
gdk start
bundle exec rails runner "Feature.enable(:knowledge_graph)"

Hit any orbit endpoint:

curl -sk --header "PRIVATE-TOKEN: $TOKEN" "https://gdk.test:3443/api/v4/orbit/schema"

Then check /-/metrics:

curl -sk https://gdk.test:3443/-/metrics | grep gitlab_knowledge_graph_

Verified output

gitlab_knowledge_graph_grpc_duration_seconds_sum{method="get_graph_schema",status="ok"} 0.006817
gitlab_knowledge_graph_grpc_duration_seconds_count{method="execute_query",status="error"} 1
gitlab_knowledge_graph_jwt_build_duration_seconds_sum 0.099997
gitlab_knowledge_graph_auth_context_duration_seconds_sum 0.096225
gitlab_knowledge_graph_redaction_duration_seconds_sum 0.198219
gitlab_knowledge_graph_redaction_batch_size_sum 1