feat: COSMOSHUB spec from MELLIFERA

relaycore.MetricsInterface had (providerAddress, chainId, apiInterface, method)
while ConsumerMetricsManagerInf had (chainId, apiInterface, providerAddress, method).
RPCConsumerLogs silently swapped the args as a bridge.

Standardise to (chainId, apiInterface, providerAddress, method) everywhere and
remove the swap from RPCConsumerLogs.

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>