Instrument connector metrics

[haasonsaas]

Summary\n- add connector operation counters and latency histograms in ASB metrics\n- record connector issue, revoke, and GitHub proxy execution operations by connector kind\n- extend metrics tests to cover issue, revoke, and proxy execution instrumentation\n\n## Testing\n- go test ./internal/app\n- go test ./internal/bootstrap ./cmd/asb-api ./internal/api/httpapi ./internal/worker\n- go test ./...\n- git diff --check\n

[cursor]

PR Summary

Low Risk
Low risk observability-only change; it adds Prometheus metrics and lightweight timing around connector Issue, Revoke, and GitHub proxy execution without changing business logic.

Overview
Adds new Prometheus metrics to track connector operation volume and latency: *_connector_operations_total (labeled by connector_kind, operation, outcome) and *_connector_operation_seconds (labeled by connector_kind, operation).

Instruments connector calls to record these metrics for grant issue, grant revoke during cleanup/state transitions, and ExecuteGitHubProxy operations. Updates metrics tests to assert the new counters/histogram observations, including a new proxy execution test.

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