Fix service pending request gauge cleanup

[taylan-oai]

What changed

• record ServicePendingRequests after request cleanup as well as admission
• keep pending-request metric updates ordered by emitted operation series when limiter buckets differ
• add regression coverage for completion, rejection, concurrent updates, and Nexus operation series

Validation

• go test -tags test_dep ./common/rpc/interceptor
• go test -tags test_dep ./service/frontend -run TestNexus
• go test -tags disable_grpc_modules,test_dep ./common/rpc/... -count=1
• make lint-code

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