add bench test for byIndexes

$ go test -run=XXX -count=5 -bench=.  sigs.k8s.io/controller-runtime/pkg/cache/internal
goos: linux
goarch: amd64
pkg: sigs.k8s.io/controller-runtime/pkg/cache/internal
cpu: Intel(R) Core(TM) i5-9600K CPU @ 3.70GHz
BenchmarkByIndexes1-4   	 4208028	       292.4 ns/op
BenchmarkByIndexes1-4   	 4045444	       286.0 ns/op
BenchmarkByIndexes1-4   	 4226229	       280.1 ns/op
BenchmarkByIndexes1-4   	 4239436	       280.1 ns/op
BenchmarkByIndexes1-4   	 4179435	       293.4 ns/op
BenchmarkByIndexes2-4   	 2602788	       486.0 ns/op
BenchmarkByIndexes2-4   	 2169660	       474.7 ns/op
BenchmarkByIndexes2-4   	 2575916	       478.9 ns/op
BenchmarkByIndexes2-4   	 2544490	       463.5 ns/op
BenchmarkByIndexes2-4   	 2524232	       484.2 ns/op
BenchmarkByIndexes3-4   	 2030055	       561.2 ns/op
BenchmarkByIndexes3-4   	 2113954	       580.8 ns/op
BenchmarkByIndexes3-4   	 1839356	       589.2 ns/op
BenchmarkByIndexes3-4   	 1685526	       610.7 ns/op
BenchmarkByIndexes3-4   	 1933444	       600.2 ns/op
BenchmarkByIndexes4-4   	 1813144	       681.9 ns/op
BenchmarkByIndexes4-4   	 1566775	       667.5 ns/op
BenchmarkByIndexes4-4   	 1807480	       658.5 ns/op
BenchmarkByIndexes4-4   	 1806310	       656.9 ns/op
BenchmarkByIndexes4-4   	 1539990	       673.1 ns/op
BenchmarkByIndexes5-4   	 1610383	       751.0 ns/op
BenchmarkByIndexes5-4   	 1344288	       774.8 ns/op
BenchmarkByIndexes5-4   	 1586659	       785.9 ns/op
BenchmarkByIndexes5-4   	 1574892	       770.8 ns/op
BenchmarkByIndexes5-4   	 1382383	       787.7 ns/op
PASS
ok  	sigs.k8s.io/controller-runtime/pkg/cache/internal	50.485s

pkg/cache/internal/cache_reader_test.go

@@ -0,0 +1,73 @@
+package internal
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"testing"
+
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/fields"
+	"k8s.io/client-go/tools/cache"
+)
+
+func benchmarkByIndexes(sel fields.Selector, b *testing.B) {
+	indexFunc := func(val string) func(obj interface{}) ([]string, error) {
+		return func(obj interface{}) ([]string, error) {
+			pod := obj.(*corev1.Pod)
+			if strings.Contains(pod.Labels["foo"], val) {
+				return []string{val}, nil
+			}
+			return nil, nil
+		}
+	}
+
+	index := cache.NewIndexer(cache.MetaNamespaceKeyFunc, cache.Indexers{
+		FieldIndexName("idx1"): indexFunc("1"),
+		FieldIndexName("idx2"): indexFunc("2"),
+		FieldIndexName("idx3"): indexFunc("3"),
+		FieldIndexName("idx4"): indexFunc("4"),
+		FieldIndexName("idx5"): indexFunc("5"),
+	})
+
+	for i := 0; i < 10000; i++ {
+		err := index.Add(&corev1.Pod{ObjectMeta: metav1.ObjectMeta{
+			Name: fmt.Sprintf("test-pod-%d", i), Labels: map[string]string{"foo": strconv.Itoa(i)}}})
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := byIndexes(index, sel.Requirements(), "")
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkByIndexes1(b *testing.B) {
+	sel := fields.ParseSelectorOrDie("idx1=1")
+	benchmarkByIndexes(sel, b)
+}
+
+func BenchmarkByIndexes2(b *testing.B) {
+	sel := fields.ParseSelectorOrDie("idx1=1,idx2=2")
+	benchmarkByIndexes(sel, b)
+}
+
+func BenchmarkByIndexes3(b *testing.B) {
+	sel := fields.ParseSelectorOrDie("idx1=1,idx2=2,idx3=3")
+	benchmarkByIndexes(sel, b)
+}
+
+func BenchmarkByIndexes4(b *testing.B) {
+	sel := fields.ParseSelectorOrDie("idx1=1,idx2=2,idx3=3,idx4=4")
+	benchmarkByIndexes(sel, b)
+}
+
+func BenchmarkByIndexes5(b *testing.B) {
+	sel := fields.ParseSelectorOrDie("idx1=1,idx2=2,idx3=3,idx4=4,idx5=5")
+	benchmarkByIndexes(sel, b)
+}