Add a series ops metric "purge_on_request"

It counts series deletions triggered via the API.

storage/local/instrumentation.go

@@ -57,6 +57,7 @@ const (
 	unarchive          = "unarchive"
 	memoryPurge        = "purge_from_memory"
 	archivePurge       = "purge_from_archive"
+	requestedPurge     = "purge_on_request"
 	memoryMaintenance  = "maintenance_in_memory"
 	archiveMaintenance = "maintenance_in_archive"
 

storage/local/storage.go

@@ -527,6 +527,7 @@ func (s *memorySeriesStorage) DropMetricsForFingerprints(fps ...model.Fingerprin
 		}
 
 		s.fpLocker.Unlock(fp)
+		s.seriesOps.WithLabelValues(requestedPurge).Inc()
 	}
 }