Refactor Labels creation

Previous Labels creation involved creating a lot more smaller objects
This change creates the final object earlier and reuses this memory.
Turns out that this change by itself does not help all that much.
But it does allow the Labels to use the insertCtx pool, which
is implemented in the next commit.

Author: cevian

pkg/pgmodel/labels.go

@@ -31,27 +31,29 @@ func EmptyLables() Labels {
 // LabelsFromSlice converts a labels.Labels to a Labels object
 func LabelsFromSlice(ls labels.Labels) (Labels, error) {
 	length := len(ls)
-	names := make([]string, 0, length)
-	values := make([]string, 0, length)
+	labels := Labels{
+		names:  make([]string, 0, length),
+		values: make([]string, 0, length),
+	}
 
-	metricName := ""
+	labels.metricName = ""
 	for _, l := range ls {
-		names = append(names, l.Name)
-		values = append(values, l.Value)
+		labels.names = append(labels.names, l.Name)
+		labels.values = append(labels.values, l.Value)
 		if l.Name == MetricNameLabelName {
-			metricName = l.Value
+			labels.metricName = l.Value
 		}
 	}
 
-	return LabelsFromSlices(names, values, metricName)
+	err := initLabels(&labels)
+	return labels, err
 }
 
-// LabelsFromSlices creates a Labels object from keys, values, and metric name
-func LabelsFromSlices(names []string, values []string, metricName string) (Labels, error) {
-	l := Labels{names: names, values: values, metricName: metricName}
+// initLabels intializes labels
+func initLabels(l *Labels) error {
 
-	if !sort.IsSorted(&l) {
-		sort.Sort(&l)
+	if !sort.IsSorted(l) {
+		sort.Sort(l)
 	}
 
 	length := len(l.names)
@@ -67,7 +69,7 @@ func LabelsFromSlices(names []string, values []string, metricName string) (Label
 	// total length anyway, we only use 16bits to store the legth of each substring
 	// in our string encoding
 	if expectedStrLen > math.MaxUint16 {
-		return l, fmt.Errorf("series too long, combined series has length %d, max length %d", expectedStrLen, ^uint16(0))
+		return fmt.Errorf("series too long, combined series has length %d, max length %d", expectedStrLen, ^uint16(0))
 	}
 
 	// the string representation is
@@ -100,26 +102,28 @@ func LabelsFromSlices(names []string, values []string, metricName string) (Label
 
 	l.str = builder.String()
 
-	return l, nil
+	return nil
 }
 
 func labelProtosToLabels(labelPairs []prompb.Label) (Labels, string, error) {
 	length := len(labelPairs)
-	names := make([]string, 0, length)
-	values := make([]string, 0, length)
+	labels := Labels{
+		names:  make([]string, 0, length),
+		values: make([]string, 0, length),
+	}
 
-	metricName := ""
+	labels.metricName = ""
 	for _, l := range labelPairs {
-		names = append(names, l.Name)
-		values = append(values, l.Value)
+		labels.names = append(labels.names, l.Name)
+		labels.values = append(labels.values, l.Value)
 		if l.Name == MetricNameLabelName {
-			metricName = l.Value
+			labels.metricName = l.Value
 		}
 	}
 
-	ls, err := LabelsFromSlices(names, values, metricName)
+	err := initLabels(&labels)
 
-	return ls, metricName, err
+	return labels, labels.metricName, err
 }
 
 func (l Labels) isEmpty() bool {