refactor(metrics): route all controllers through core/metrics helpers

[behinddwalls]

Summary

Route every controller and the consumer framework through the core/metrics helpers so metric emission is consistent across the codebase.

What changed

Migrated to metrics.Begin / op.Complete + metrics.NamedCounter:

• RPC controllers: gateway/controller/land.go, orchestrator/controller/ping.go, stovepipe/controller/ping.go
• Queue controllers: orchestrator/controller/{batch,build,start,score,speculate,buildsignal,log}/*.go
• Extension: extension/mergechecker/github/checker.go

Each migrated method now wraps its work with op := metrics.Begin(scope, opName); defer func() { op.Complete(retErr) }(), which emits {scope}.{op}.called, {op}.succeeded / {op}.failed, {op}.latency, and {op}.latency_histogram with automatic error_origin, retryable, and dependency classification tags via core/errs. Sub-event counters (deserialize_errors, storage_errors, publish_errors, …) go through metrics.NamedCounter so they share the same sub-scope prefix.

Consumer framework — core/consumer/consumer.go:
Migrated to metrics.NamedCounter / metrics.NamedTimer rather than Begin/Complete because the framework needs custom success=true|false|cancel and operation=ack|nack tags that the standard Op lifecycle can't express.

Op-name de-duplication:
Each migrated method declares const opName = "..." once and references it from every emit (file-level in speculate.go where emits span six helper methods).

Wire-format change

Metric names move from flat keys to {controller_subscope}.{op}.{event}:

Before	After
land_request_count	land_controller.land.called
land_request_latency	land_controller.land.latency
received (per controller)	{controller}_controller.process.called
processed	{controller}_controller.process.succeeded
deserialize_errors	{controller}_controller.process.deserialize_errors
messages_received (consumer)	consumer.process.messages_received
controller_latency (consumer)	consumer.process.controller_latency
ack_count, nack_count, …	consumer.process.ack_count, consumer.process.nack_count, …

Dashboards keyed on the old flat names will need updating; the success=true|false|cancel, operation=ack|nack, and per-controller tags on consumer metrics are preserved.

Test Plan

• make build — all 135 targets build clean.
• make test — all 34 test targets pass, including:
  • //core/consumer:consumer_test — verifies the migrated processDelivery still emits controller_latency (with success=true|false) and ack_count (substring matches survive the process. prefix).
  • //extension/mergechecker/github:github_test
  • //gateway/controller:controller_test, //orchestrator/controller:controller_test, //stovepipe/controller:controller_test
  • Per-stage controllers: batch, build, buildsignal, log, score, speculate, start
• make gazelle, make fmt, make mocks, make tidy — re-running on top of the diff produces zero further changes.
• make lint-license — clean.
• No test assertions on the renamed metrics needed updating (consumer_test.go uses strings.Contains, which still matches the new fully-qualified names).

Issues

None.