Add Prometheus metrics package with HTTP middleware and storage instrumentation
- internal/metrics/metrics.go: defines all metric variables (http, scheduler,
provider, storage, build info) using promauto for zero-config registration
- internal/metrics/http.go: Gin middleware recording request count, duration,
and in-flight gauge using c.FullPath() to avoid high-cardinality labels
- internal/app/instrumented_storage.go: InstrumentedStorage wrapper implementing
storage.Storage, recording operation counts and durations for all entities
Pierre-Olivier Mercier
b7beefed3f