compactor: fix metric name for a compactor (#11412)

**What this PR does / why we need it**:
I added a new metric but prefixed it by mistake with
`loki_boltdb_shipper` instead of `loki_compactor`. This PR fixes it.

pkg/compactor/metrics.go

@@ -49,7 +49,7 @@ func newMetrics(r prometheus.Registerer) *metrics {
  Help: "Time (in seconds) spent in applying retention",
  }),
  applyRetentionLastSuccess: promauto.With(r).NewGauge(prometheus.GaugeOpts{
- Namespace: "loki_boltdb_shipper",
+ Namespace: "loki_compactor",
  Name: "apply_retention_last_successful_run_timestamp_seconds",
  Help: "Unix timestamp of the last successful retention run",
  }),