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   	    1885	    579264 ns/op
BenchmarkByIndexes1-4   	    1977	    579334 ns/op
BenchmarkByIndexes1-4   	    1920	    583148 ns/op
BenchmarkByIndexes1-4   	    1958	    583714 ns/op
BenchmarkByIndexes1-4   	    1932	    592758 ns/op
BenchmarkByIndexes2-4   	     688	   1730501 ns/op
BenchmarkByIndexes2-4   	     706	   1693914 ns/op
BenchmarkByIndexes2-4   	     697	   1740027 ns/op
BenchmarkByIndexes2-4   	     686	   1669624 ns/op
BenchmarkByIndexes2-4   	     691	   1736906 ns/op
BenchmarkByIndexes3-4   	     562	   2007959 ns/op
BenchmarkByIndexes3-4   	     591	   1998279 ns/op
BenchmarkByIndexes3-4   	     576	   1992566 ns/op
BenchmarkByIndexes3-4   	     584	   2039316 ns/op
BenchmarkByIndexes3-4   	     588	   1981963 ns/op
BenchmarkByIndexes4-4   	     536	   2090786 ns/op
BenchmarkByIndexes4-4   	     565	   2084039 ns/op
BenchmarkByIndexes4-4   	     562	   2114272 ns/op
BenchmarkByIndexes4-4   	     514	   2095443 ns/op
BenchmarkByIndexes4-4   	     565	   2179949 ns/op
BenchmarkByIndexes5-4   	     555	   2162249 ns/op
BenchmarkByIndexes5-4   	     556	   2079339 ns/op
BenchmarkByIndexes5-4   	     544	   2108629 ns/op
BenchmarkByIndexes5-4   	     568	   2044934 ns/op
BenchmarkByIndexes5-4   	     568	   2097770 ns/op
PASS
ok  	sigs.k8s.io/controller-runtime/pkg/cache/internal	49.990s

pkg/cache/internal/cache_reader_test.go

@@ -0,0 +1,77 @@
+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) {
+	requires := sel.Requirements()
+	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{KeyToNamespacedKey("", 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 < 15000; 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++ {
+		objs, err := byIndexes(index, requires, "")
+		if err != nil {
+			b.Fatal(err)
+		}
+		if len(objs) == 0 {
+			b.Fatal("expect to match objs, got 0")
+		}
+	}
+}
+
+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)
+}