expose compactor metrics

Signed-off-by: Ben Ye <benye@amazon.com>

tsdb/compact.go

@@ -96,7 +96,8 @@ type CompactorMetrics struct {
 	ChunkRange        prometheus.Histogram
 }
 
-func newCompactorMetrics(r prometheus.Registerer) *CompactorMetrics {
+// NewCompactorMetrics initializes metrics for Compactor.
+func NewCompactorMetrics(r prometheus.Registerer) *CompactorMetrics {
 	m := &CompactorMetrics{}
 
 	m.Ran = prometheus.NewCounter(prometheus.CounterOpts{
@@ -203,7 +204,7 @@ func NewLeveledCompactorWithOptions(ctx context.Context, r prometheus.Registerer
 		ranges:                      ranges,
 		chunkPool:                   pool,
 		logger:                      l,
-		metrics:                     newCompactorMetrics(r),
+		metrics:                     NewCompactorMetrics(r),
 		ctx:                         ctx,
 		maxBlockChunkSegmentSize:    maxBlockChunkSegmentSize,
 		mergeFunc:                   mergeFunc,