Instrument artifact metrics

[haasonsaas]

Summary\n- add active artifact gauges by connector kind and unwrap counters by connector kind\n- update grant, unwrap, revoke, and cleanup flows to keep artifact gauges accurate\n- extend metrics tests to cover artifact issuance, unwraps, and artifact teardown transitions\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: changes are limited to Prometheus instrumentation, adding new collectors and updating a few lifecycle paths to keep gauge counts consistent.

Overview
Adds Prometheus metrics for artifact lifecycle visibility: a new *_artifacts_active gauge (labeled by connector_kind) and a new *_artifact_unwraps_total counter.

Wires these metrics into artifact issuance (issueGrant), artifact use (UnwrapArtifact), grant revoke/expire transitions, and cleanup-driven artifact expiration so the active-artifact gauge decrements/ increments based on ArtifactState transitions.

Extends metrics_test.go to assert artifact gauge updates on issuance and teardown, and adds coverage for unwrap counting and active-artifact decrements on unwrap.

^{Reviewed by Cursor Bugbot for commit 253fe87. Bugbot is set up for automated code reviews on this repo. Configure here.}