cli/command/container: add BenchmarkStatsFormat()

To test:

    GO111MODULE=off go test -test.v -test.bench '^BenchmarkStatsFormat' -test.run '^$' ./cli/command/container/
    goos: darwin
    goarch: amd64
    pkg: github.com/docker/cli/cli/command/container
    cpu: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
    BenchmarkStatsFormat
    BenchmarkStatsFormat-8   	    2482	    522721 ns/op	   62439 B/op	    5600 allocs/op
    PASS
    ok  	github.com/docker/cli/cli/command/container	1.369s

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/formatter_stats_test.go

@@ -308,3 +308,38 @@ func TestContainerStatsContextWriteTrunc(t *testing.T) {
 		out.Reset()
 	}
 }
+
+func BenchmarkStatsFormat(b *testing.B) {
+	b.ReportAllocs()
+	stats := genStats()
+
+	for i := 0; i < b.N; i++ {
+		for _, s := range stats {
+			_ = s.CPUPerc()
+			_ = s.MemUsage()
+			_ = s.MemPerc()
+			_ = s.NetIO()
+			_ = s.BlockIO()
+			_ = s.PIDs()
+		}
+	}
+}
+
+func genStats() []statsContext {
+	entry := statsContext{s: StatsEntry{
+		CPUPercentage:    12.3456789,
+		Memory:           123.456789,
+		MemoryLimit:      987.654321,
+		MemoryPercentage: 12.3456789,
+		BlockRead:        123.456789,
+		BlockWrite:       987.654321,
+		NetworkRx:        123.456789,
+		NetworkTx:        987.654321,
+		PidsCurrent:      123456789,
+	}}
+	stats := make([]statsContext, 100)
+	for i := 0; i < 100; i++ {
+		stats = append(stats, entry)
+	}
+	return stats
+}