…ssue #14)

The field was written in SetProcessingTimestampAfterRelay but never read by
any production code path. Remove the field, strip the write from the setter,
and update the test to only assert ProcessingTimestamp behaviour.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Addresses all issues raised in code review (issues #1–9, #11–19):

Correctness:
- Add apiInterface label to endpointSelectionScore gauge and propagate it
  through SetProviderSelected interface and all call sites (#4)
- Align SetRelayNodeErrorMetric parameter order to (chainId, apiInterface,
  providerAddress, method) — was (providerAddress, chainId, apiInterface) (#7)
- Scope metric setters outside the lock in RegisterEndpoint to avoid
  re-entrant RWMutex deadlock (#11)
- Record cache latency histogram only on cache hits, not misses (#12)
- Remove dead field MeasureAfterProviderProcessingTime from RelayMetrics
  and its sole write site (#14)

Dead code:
- Remove dead stubs SetRelaySentToProviderMetric and SetRequestPerProvider
  from SmartRouterMetricsManager (#5)
- Delete empty consumer_metrics_manager_test.go (#18)

Simplification:
- Replace bespoke registerMetric closure with generic registerOrReuse
  helper throughout ConsumerMetricsManager (#6)
- Unify latency histogram buckets via shared LatencyBuckets variable in
  buckets.go — removes per-file duplicates (#8)
- Drop unnecessary goroutine spawns from RPCConsumerLogs metric forwarders;
  callers already run in goroutines where needed (#9)
- Replace RequestProperties struct with *RelayMetrics in RecordDirectRelayEnd,
  eliminating a duplicate of the same fields (#15)

Documentation:
- Document the request-group counting invariants in SetRelayMetrics (#13)
- Add comment explaining why UpdateHealthcheckStatusBreakdown is a no-op
  in SmartRouterMetricsManager (#16)
- Fix struct comment to list all four labels including apiInterface (#17)

Tests:
- Scope gomock.Controller to each subtest in TestIsArchiveRequest,
  TestIsDebugOrTraceRequest, and TestIsBatchRequest (#19)
- Add TestConsumerSetRelayMetrics_PartitionInvariant asserting
  batch+read+write==total and success+failed==total (#13)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>