Do not parse string of empty matchers (#5980)

This fixes a bug in the index gateway when querying values for a label.
Since the gRPC handler for LabelValuesForMetricName in the index gateway
allows empty matchers in the LabelValuesForMetricNameRequest, we need to
check if the matchers string is an empty matcher (`{}`) before we parse
the string.

This bug was introduced with the index gateway api refactoring in #5892

Fixes: #5965

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

pkg/logql/syntax/parser.go

@@ -15,7 +15,11 @@ import (
 	"github.com/grafana/loki/pkg/util"
 )
 
-const errAtleastOneEqualityMatcherRequired = "queries require at least one regexp or equality matcher that does not have an empty-compatible value. For instance, app=~\".*\" does not meet this requirement, but app=~\".+\" will"
+const (
+	EmptyMatchers = "{}"
+
+	errAtleastOneEqualityMatcherRequired = "queries require at least one regexp or equality matcher that does not have an empty-compatible value. For instance, app=~\".*\" does not meet this requirement, but app=~\".+\" will"
+)
 
 var parserPool = sync.Pool{
 	New: func() interface{} {

pkg/storage/stores/shipper/indexgateway/gateway.go

@@ -333,9 +333,14 @@ func (g *Gateway) LabelValuesForMetricName(ctx context.Context, req *indexgatewa
 	if err != nil {
 		return nil, err
 	}
-	matchers, err := syntax.ParseMatchers(req.Matchers)
-	if err != nil {
-		return nil, err
+	var matchers []*labels.Matcher
+	// An empty matchers string cannot be parsed,
+	// therefore we check the string representation of the the matchers.
+	if req.Matchers != syntax.EmptyMatchers {
+		matchers, err = syntax.ParseMatchers(req.Matchers)
+		if err != nil {
+			return nil, err
+		}
 	}
 	names, err := g.indexQuerier.LabelValuesForMetricName(ctx, instanceID, req.From, req.Through, req.MetricName, req.LabelName, matchers...)
 	if err != nil {