Don't increment prometheus_tsdb_compactions_failed_total when context is canceled (#10772)

When restarting Prometheus I sometimes see:

caller=db.go:832 level=error component=tsdb msg="compaction failed" err="compact head: persist head block: 2 errors: populate block: context canceled; context canceled"

And prometheus_tsdb_compactions_failed_total metric gets incremented.
This makes it more difficult to write alerts based on
prometheus_tsdb_compactions_failed_total metric since any restart can
trigger it.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

tsdb/db.go

@@ -888,7 +888,9 @@ func (db *DB) Compact() (returnErr error) {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
 	defer func() {
-		if returnErr != nil {
+		if returnErr != nil && !errors.Is(returnErr, context.Canceled) {
+			// If we got an error because context was canceled then we're most likely
+			// shutting down TSDB and we don't need to report this on metrics
 			db.metrics.compactionsFailed.Inc()
 		}
 	}()