feat(metrics): add Prometheus GC metrics (#3863)

* feat(metrics): add Prometheus GC metrics

Track garbage collection activity with three new metrics:
- zot_gc_runs_total (counter, label: error) — GC run count
- zot_gc_duration_seconds (summary) — GC run duration
- zot_gc_deleted_total (counter, label: type) — items deleted
  by type: blob, manifest, upload

MetricServer is added to GarbageCollect and wired through
all callers (controller, verify-feature retention, tests).

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(test): add missing metrics var in GCS GC tests

TestGCSGarbageCollectImageIndex and
TestGCSGarbageCollectChainedImageIndexes were missing the
metrics variable required by NewGarbageCollect after the
MetricServer parameter was added.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(test): add defer metrics.Stop() in GC tests

Prevent goroutine/port leaks by stopping MetricsServer in
storage_test.go (3 functions) and gcs_test.go (also add
missing metrics declaration in TestGCSGarbageCollectImageManifest).

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(test): cover `CleanRepo` error path

Add test that exercises the error branch in
`CleanRepo` where `cleanRepo` fails, covering
the metrics calls and log lines flagged by Codecov.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* test: Cover GC error paths for codecov

Add three tests in gc_internal_test.go to cover previously
untested error branches in `removeBlobUploads` and
`removeUnreferencedBlobs`: `ListBlobUploads` failure,
`addIndexBlobsToReferences` failure, and `PathNotFoundError`
from `GetAllBlobs`.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* test(gc): cover remaining error paths

Cover `StatBlobUpload`, `digest.Validate()`,
`isBlobOlderThan`, and `CleanupRepo` error branches
in `removeBlobUploads` and `removeUnreferencedBlobs`.

`removeUnreferencedBlobs` now at 100% coverage,
`removeBlobUploads` from 78.3% to 91.3%.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* test: cover `sanityChecks` label name mismatch

Try to avoid -0.09% coverage regression on `minimal.go`
by exercising the uncovered branch in `sanityChecks`
where label names have correct count but wrong values.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* test(gc): exercise real GC path in metrics test

TestGCMetrics was calling metric helpers directly instead of
running actual garbage collection, so it couldn't catch wiring
regressions where `CleanRepo` stops recording metrics.

Now uploads an orphaned blob and runs `gc.CleanRepo` end-to-end,
verifying metrics appear on the Prometheus endpoint.

Suggestion from Copilot: https://github.com/project-zot/zot/pull/3863#discussion_r3129324719

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(gc): skip deletion metrics when DryRun is enabled

https://github.com/project-zot/zot/pull/3863#discussion_r3129324684

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(test): stop leaked MetricsServer goroutines in GCS tests

https://github.com/project-zot/zot/pull/3863#discussion_r3129324657

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* refactor(test): drop unnecessary zlog import alias

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(monitoring): expose metric types outside build tag

`MetricsCopy` and related types were only visible under `\!metrics`,
causing a typecheck failure when golangci-lint runs with `-tags metrics`.
Moving the type definitions to `common.go` makes them unconditionally available.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* fix(monitoring): remove extra blank line for gci

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* test(gc): cover both dry-run and real deletion metrics

And fix issue with build tag with metrics

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* Satisfy testpackage linter for gc metrics test

The `testpackage` linter allows `package gc` only in files named
`*_internal_test.go`; rename to follow that convention.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

---------

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

pkg/api/controller.go

@@ -557,7 +557,7 @@ func (c *Controller) StartBackgroundTasks() {
 	}
 
 	// Run GC and retention tasks
-	RunGCTasks(c.Config, c.StoreController, c.MetaDB, c.taskScheduler, c.Log, c.Audit)
+	RunGCTasks(c.Config, c.StoreController, c.MetaDB, c.taskScheduler, c.Log, c.Audit, c.Metrics)
 
 	// Enable running dedupe blobs both ways (dedupe or restore deduped blobs)
 	c.StoreController.DefaultStore.RunDedupeBlobs(time.Duration(0), c.taskScheduler)
@@ -617,7 +617,7 @@ func (c *Controller) StartBackgroundTasks() {
 
 // RunGCTasks runs minimal GC and retention tasks without full controller.
 func RunGCTasks(conf *config.Config, storeController storage.StoreController, metaDB mTypes.MetaDB,
-	taskScheduler *scheduler.Scheduler, logger log.Logger, audit *log.Logger,
+	taskScheduler *scheduler.Scheduler, logger log.Logger, audit *log.Logger, metrics monitoring.MetricServer,
 ) {
 	// Enable running garbage-collect periodically for DefaultStore
 	storageConfig := conf.CopyStorageConfig()
@@ -626,7 +626,7 @@ func RunGCTasks(conf *config.Config, storeController storage.StoreController, me
 			Delay:             storageConfig.GCDelay,
 			ImageRetention:    storageConfig.Retention,
 			MaxSchedulerDelay: storageConfig.GCMaxSchedulerDelay,
-		}, audit, logger)
+		}, audit, logger, metrics)
 
 		gc.CleanImageStorePeriodically(storageConfig.GCInterval, taskScheduler)
 	}
@@ -641,7 +641,7 @@ func RunGCTasks(conf *config.Config, storeController storage.StoreController, me
 						Delay:             subStorageConfig.GCDelay,
 						ImageRetention:    subStorageConfig.Retention,
 						MaxSchedulerDelay: subStorageConfig.GCMaxSchedulerDelay,
-					}, audit, logger)
+					}, audit, logger, metrics)
 
 				gc.CleanImageStorePeriodically(subStorageConfig.GCInterval, taskScheduler)
 			}