[PERF] PromQL: only reset labels builder when needed

Signed-off-by: Linas Medziunas <linas.medziunas@gmail.com>

promql/engine.go

@@ -2951,7 +2951,6 @@ func resultMetric(lhs, rhs labels.Labels, op parser.ItemType, matching *parser.V
 		enh.resultMetric = make(map[string]labels.Labels, len(enh.Out))
 	}
 
-	enh.resetBuilder(lhs)
 	buf := bytes.NewBuffer(enh.lblResultBuf[:0])
 	enh.lblBuf = lhs.Bytes(enh.lblBuf)
 	buf.Write(enh.lblBuf)
@@ -2964,6 +2963,7 @@ func resultMetric(lhs, rhs labels.Labels, op parser.ItemType, matching *parser.V
 	}
 	str := string(enh.lblResultBuf)
 
+	enh.resetBuilder(lhs)
 	if changesMetricSchema(op) {
 		// Setting empty Metadata causes the deletion of those if they exists.
 		schema.Metadata{}.SetToLabels(enh.lb)