Simplify and extend the benchmarks of timing histograms

Replace the use of a clock abstraction with direct provision of the
time-reading function.

Add benchmarks of two ways to fetch and use a vector member.

Following are the results using the revised benchmark suite.

(base) mspreitz@mjs12 prometheusextension % go test  -benchmem -run=^$ -bench Histogram .
goos: darwin
goarch: amd64
pkg: k8s.io/component-base/metrics/prometheusextension
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkTimingHistogramDirect-16           	28562178	        39.47 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltCached-16     	29268819	        39.41 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltFetched-16    	 8345041	       142.5 ns/op	      32 B/op	       1 allocs/op
BenchmarkWeightedHistogram-16               	49803134	        24.22 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogram-16                       	41756210	        29.40 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	k8s.io/component-base/metrics/prometheusextension	6.488s

(base) mspreitz@mjs12 prometheusextension % go test  -benchmem -run=^$ -bench Histogram .
goos: darwin
goarch: amd64
pkg: k8s.io/component-base/metrics/prometheusextension
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkTimingHistogramDirect-16           	28246786	        39.60 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltCached-16     	29318173	        39.44 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltFetched-16    	 8441341	       142.5 ns/op	      32 B/op	       1 allocs/op
BenchmarkWeightedHistogram-16               	48469184	        24.21 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogram-16                       	41001742	        29.51 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	k8s.io/component-base/metrics/prometheusextension	6.340s

(base) mspreitz@mjs12 prometheusextension % go test  -benchmem -run=^$ -bench Histogram .
goos: darwin
goarch: amd64
pkg: k8s.io/component-base/metrics/prometheusextension
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkTimingHistogramDirect-16           	28983674	        40.82 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltCached-16     	29527742	        43.83 ns/op	       0 B/op	       0 allocs/op
BenchmarkTimingHistogramVecEltFetched-16    	 8238685	       143.8 ns/op	      32 B/op	       1 allocs/op
BenchmarkWeightedHistogram-16               	49594070	        24.24 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogram-16                       	41100378	        29.35 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	k8s.io/component-base/metrics/prometheusextension	6.554s

Kubernetes-commit: f3068efcfb9f61586dcfae4b87bd084628d53428

metrics/prometheusextension/timing_histogram_test.go

@@ -202,8 +202,7 @@ func (ufc *unsyncFakeClock) SetTime(now time.Time) {
 func BenchmarkTimingHistogramDirect(b *testing.B) {
 	b.StopTimer()
 	now := time.Now()
-	clk := &unsyncFakeClock{now: now}
-	hist, err := NewTestableTimingHistogram(clk.Now, TimingHistogramOpts{
+	hist, err := NewTestableTimingHistogram(func() time.Time { return now }, TimingHistogramOpts{
 		Namespace: "testns",
 		Subsystem: "testsubsys",
 		Name:      "testhist",
@@ -216,8 +215,48 @@ func BenchmarkTimingHistogramDirect(b *testing.B) {
 	var x int
 	b.StartTimer()
 	for i := 0; i < b.N; i++ {
-		clk.now = clk.now.Add(time.Duration(31-x) * time.Microsecond)
+		now = now.Add(time.Duration(31-x) * time.Microsecond)
 		hist.Set(float64(x))
 		x = (x + i) % 23
 	}
 }
+func BenchmarkTimingHistogramVecEltCached(b *testing.B) {
+	b.StopTimer()
+	now := time.Now()
+	vec := NewTestableTimingHistogramVec(func() time.Time { return now }, TimingHistogramOpts{
+		Namespace: "testns",
+		Subsystem: "testsubsys",
+		Name:      "testhist",
+		Help:      "Me",
+		Buckets:   []float64{1, 2, 4, 8, 16},
+	},
+		"label1", "label2")
+	hist := vec.WithLabelValues("val1", "val2")
+	var x int
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		now = now.Add(time.Duration(31-x) * time.Microsecond)
+		hist.Set(float64(x))
+		x = (x + i) % 23
+	}
+}
+
+func BenchmarkTimingHistogramVecEltFetched(b *testing.B) {
+	b.StopTimer()
+	now := time.Now()
+	vec := NewTestableTimingHistogramVec(func() time.Time { return now }, TimingHistogramOpts{
+		Namespace: "testns",
+		Subsystem: "testsubsys",
+		Name:      "testhist",
+		Help:      "Me",
+		Buckets:   []float64{1, 2, 4, 8, 16},
+	},
+		"label1", "label2")
+	var x int
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		now = now.Add(time.Duration(31-x) * time.Microsecond)
+		vec.WithLabelValues("val1", "val2").Set(float64(x))
+		x = (x + i) % 23
+	}
+}