Use cached label sets

The distributed engine retrieves label sets once per query, and
doing the expensive copying and conversion uses a lot of memory.

We already set them in the format we need in the endpoint status,
so we can retrieve them from there.

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

pkg/query/endpointset.go

@@ -792,7 +792,7 @@ func (er *endpointRef) LabelSets() []labels.Labels {
 	er.mtx.RLock()
 	defer er.mtx.RUnlock()
 
-	return er.labelSets()
+	return er.status.LabelSets
 }
 
 func (er *endpointRef) labelSets() []labels.Labels {