Preallocate Labels in labels.Builder (#10749)

This tries to avoid re-allocations of labels slice since we know possible max size

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

model/labels/labels.go

@@ -349,7 +349,7 @@ func FromStrings(ss ...string) Labels {
 	if len(ss)%2 != 0 {
 		panic("invalid number of strings")
 	}
-	var res Labels
+	res := make(Labels, 0, len(ss)/2)
 	for i := 0; i < len(ss); i += 2 {
 		res = append(res, Label{Name: ss[i], Value: ss[i+1]})
 	}
@@ -452,7 +452,7 @@ func (b *Builder) Labels() Labels {
 
 	// In the general case, labels are removed, modified or moved
 	// rather than added.
-	res := make(Labels, 0, len(b.base))
+	res := make(Labels, 0, len(b.base)+len(b.add))
 Outer:
 	for _, l := range b.base {
 		for _, n := range b.del {

promql/engine_test.go

@@ -16,6 +16,7 @@ package promql
 import (
 	"context"
 	"errors"
+	"fmt"
 	"os"
 	"sort"
 	"testing"
@@ -680,7 +681,6 @@ load 10s
 			Result: Matrix{
 				Series{
 					Points: []Point{{V: 1, T: 0}, {V: 1, T: 1000}, {V: 1, T: 2000}},
-					Metric: labels.FromStrings(),
 				},
 			},
 			Start:    time.Unix(0, 0),
@@ -717,7 +717,8 @@ load 10s
 		},
 	}
 
-	for _, c := range cases {
+	for i, c := range cases {
+		t.Run(fmt.Sprintf("%d query=%s", i, c.Query), func(t *testing.T) {
 			var err error
 			var qry Query
 			if c.Interval == 0 {
@@ -730,11 +731,12 @@ load 10s
 			res := qry.Exec(test.Context())
 			if c.ShouldError {
 				require.Error(t, res.Err, "expected error for the query %q", c.Query)
-			continue
+				return
 			}
 
 			require.NoError(t, res.Err)
 			require.Equal(t, c.Result, res.Value, "query %q failed", c.Query)
+		})
 	}
 }