Add Client.Scope

[martin-sucha]

Currently the Client allocates a new context every time a counter/gauge/set is updated.

I use a small number of tags in my application multiple times, so I would like to cache the context to avoid an allocation.

This commit implements a new Scope structure that caches the context string and allows the application to update counters without an allocation.

I run the following benchmarks two times with -count=5:

func BenchmarkStatsdCounter(b *testing.B) {
	s, err := statsd.New("localhost:8125", statsd.WithNamespace("my_namespace"),
		statsd.WithClientSideAggregation(), statsd.WithExtendedClientSideAggregation())
	if err != nil {
		b.Fatal(err)
	}
	b.ReportAllocs()
	labels := []string{"label1:value1", "label2:value2"}
	b.ResetTimer()
	b.RunParallel(func(pb *testing.PB) {
		for pb.Next() {
			s.Count("my_metric", 1, labels, 1)
		}
	})
}

func BenchmarkStatsdCounterScope(b *testing.B) {
	s, err := statsd.New("localhost:8125", statsd.WithNamespace("my_namespace"),
		statsd.WithClientSideAggregation(), statsd.WithExtendedClientSideAggregation())
	if err != nil {
		b.Fatal(err)
	}
	b.ReportAllocs()
	labels := []string{"label1:value1", "label2:value2"}
	mc := s.Scope("my_metric", labels)
	b.ResetTimer()
	b.RunParallel(func(pb *testing.PB) {
		for pb.Next() {
			mc.Count(1, 1)
		}
	})
}

The benchstat results:

goos: linux
goarch: amd64
pkg: gitlab.skypicker.com/go/pkg/monitoring/metricsv2
cpu: 12th Gen Intel(R) Core(TM) i7-1260P
                      │  bench.txt   │
                      │    sec/op    │
StatsdCounter           56.25n ±  5%
StatsdCounter-2         90.99n ±  5%
StatsdCounter-4         101.2n ± 40%
StatsdCounter-8         139.9n ±  4%
StatsdCounter-16        123.6n ±  4%
StatsdCounterScope      30.40n ±  5%
StatsdCounterScope-2    107.2n ±  7%
StatsdCounterScope-4    104.5n ±  7%
StatsdCounterScope-8    126.7n ±  5%
StatsdCounterScope-16   96.54n ± 25%
geomean                 90.60n

                      │  bench.txt   │
                      │     B/op     │
StatsdCounter           48.00 ± 0%
StatsdCounter-2         48.00 ± 0%
StatsdCounter-4         48.00 ± 0%
StatsdCounter-8         48.00 ± 0%
StatsdCounter-16        48.00 ± 0%
StatsdCounterScope      0.000 ± 0%
StatsdCounterScope-2    0.000 ± 0%
StatsdCounterScope-4    0.000 ± 0%
StatsdCounterScope-8    0.000 ± 0%
StatsdCounterScope-16   0.000 ± 0%
geomean                            ¹
¹ summaries must be >0 to compute geomean

                      │  bench.txt   │
                      │  allocs/op   │
StatsdCounter           1.000 ± 0%
StatsdCounter-2         1.000 ± 0%
StatsdCounter-4         1.000 ± 0%
StatsdCounter-8         1.000 ± 0%
StatsdCounter-16        1.000 ± 0%
StatsdCounterScope      0.000 ± 0%
StatsdCounterScope-2    0.000 ± 0%
StatsdCounterScope-4    0.000 ± 0%
StatsdCounterScope-8    0.000 ± 0%
StatsdCounterScope-16   0.000 ± 0%
geomean                            ¹
¹ summaries must be >0 to compute geomean