GODRIVER-2627 optimize latency selector

[isopov]

Summary

Optimize allocations in latency server selector.

Background & Motivation

Similar to #1107 i try to optimize allocations in latency server selector. Benchmark provided without changes:

BenchmarkLatencySelector
BenchmarkLatencySelector/AllFit
BenchmarkLatencySelector/AllFit-10         	   64196	     17866 ns/op	  111273 B/op	       9 allocs/op
BenchmarkLatencySelector/AllButOneFit
BenchmarkLatencySelector/AllButOneFit-10   	   71056	     17038 ns/op	  111273 B/op	       9 allocs/op
BenchmarkLatencySelector/HalfFit
BenchmarkLatencySelector/HalfFit-10        	  124551	      9748 ns/op	   53928 B/op	       8 allocs/op
BenchmarkLatencySelector/OneFit
BenchmarkLatencySelector/OneFit-10         	 1471053	       825.6 ns/op	    1320 B/op	       3 allocs/op
PASS

And with proposed optimization:

BenchmarkLatencySelector
BenchmarkLatencySelector/AllFit
BenchmarkLatencySelector/AllFit-10         	 1868062	       686.8 ns/op	     904 B/op	       2 allocs/op
BenchmarkLatencySelector/AllButOneFit
BenchmarkLatencySelector/AllButOneFit-10   	  155904	      7382 ns/op	   41864 B/op	       3 allocs/op
BenchmarkLatencySelector/HalfFit
BenchmarkLatencySelector/HalfFit-10        	  277989	      4409 ns/op	   21384 B/op	       3 allocs/op
BenchmarkLatencySelector/OneFit
BenchmarkLatencySelector/OneFit-10         	 1247785	       966.7 ns/op	    1800 B/op	       3 allocs/op
PASS

[prestonvasquez]

@isopov thank you for the PR!

[prestonvasquez]

Instead of using a closure and constructing sub-benchmarks for each case, can we iterate over the benchmark cases? Something like this:

	for _, bcase := range []struct {
		name        string
		serversHook func(servers []description.Server)
	}{
		{
			"AllFit",
			func(servers []description.Server) {},
		},
		{
			"AllButOneFit",
			func(servers []description.Server) {
				servers[0].AverageRTT = 2 * time.Second
			},
		},
		// Continue test cases
	} {
		bcase := bcase

		b.Run(bcase.name, func(b *testing.B) {
			// your benchmark logic
		})
	}

[isopov]

Changed benchmark this way

[prestonvasquez]

LGTM

[matthewdale]

@isopov thanks for the improvements! Looks good 👍