Log Ingester.LabelNames() matchers in traces (#9544)

Signed-off-by: Marco Pracucci <marco@pracucci.com>

pkg/ingester/ingester.go

@@ -1741,6 +1741,9 @@ func (i *Ingester) LabelNames(ctx context.Context, req *client.LabelNamesRequest
 	}
 	defer func() { finishReadRequest(err) }()
 
+	spanlog, ctx := spanlogger.NewWithLogger(ctx, i.logger, "Ingester.LabelNames")
+	defer spanlog.Finish()
+
 	userID, err := tenant.TenantID(ctx)
 	if err != nil {
 		return nil, err
@@ -1768,6 +1771,9 @@ func (i *Ingester) LabelNames(ctx context.Context, req *client.LabelNamesRequest
 	}
 	defer q.Close()
 
+	// Log the actual matchers passed down to TSDB. This can be useful for troubleshooting purposes.
+	spanlog.DebugLog("num_matchers", len(matchers), "matchers", util.LabelMatchersToString(matchers))
+
 	hints := &storage.LabelHints{}
 	names, _, err := q.LabelNames(ctx, hints, matchers...)
 	if err != nil {