testing: add Elapsed method to testing.B

Elapsed returns the measured elapsed time of the benchmark,
but does not change the running state of the timer.

Fixes #43620.

Change-Id: Idd9f64c4632518eec759d2ffccbf0050d84fcc03
Reviewed-on: https://go-review.googlesource.com/c/go/+/420254
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: hopehook <hopehook@qq.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

api/next/43620.txt

@@ -0,0 +1 @@
+pkg testing, method (*B) Elapsed() time.Duration #43620

src/testing/benchmark.go

@@ -337,6 +337,17 @@ func (b *B) launch() {
 	b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
 }
 
+// Elapsed returns the measured elapsed time of the benchmark.
+// The duration reported by Elapsed matches the one measured by
+// StartTimer, StopTimer, and ResetTimer.
+func (b *B) Elapsed() time.Duration {
+	d := b.duration
+	if b.timerOn {
+		d += time.Since(b.start)
+	}
+	return d
+}
+
 // ReportMetric adds "n unit" to the reported benchmark results.
 // If the metric is per-iteration, the caller should divide by b.N,
 // and by convention units should end in "/op".

src/testing/benchmark_test.go

@@ -176,5 +176,8 @@ func ExampleB_ReportMetric() {
 		// This metric is per-operation, so divide by b.N and
 		// report it as a "/op" unit.
 		b.ReportMetric(float64(compares)/float64(b.N), "compares/op")
+		// This metric is per-time, so divide by b.Elapsed and
+		// report it as a "/ns" unit.
+		b.ReportMetric(float64(compares)/float64(b.Elapsed().Nanoseconds()), "compares/ns")
 	})
 }