stats/opentelemetry: Optimize slice allocations

[ash2k]

First commit moves attribute construction under if to only construct them when they are used.

Second commit uses WithAttributeSet() instead of WithAttributes(). This avoids copying the slice, which is not necessary for this code. See https://github.com/open-telemetry/opentelemetry-go/blob/metric/v1.28.0/metric/instrument.go#L349-L368.

Also, a bit of allocations can be removed by constructing the vararg slice one time vs when calling Record() each time.

var sink []string

func abc(vals ...string) {
	sink = vals
}

func BenchmarkFuncCall(b *testing.B) {
	b.ReportAllocs()
	b.ResetTimer()
	arg := []string{"a", "b", "c"}
	for i := 0; i < b.N; i++ {
		abc(arg...)
	}
}

The above benchmark gives me these results:

// Pass "a", "b", "c"
BenchmarkFuncCall-10    	43171615	        27.14 ns/op	      48 B/op	       1 allocs/op

// Pass arg...
BenchmarkFuncCall-10    	1000000000	         0.9530 ns/op	       0 B/op	       0 allocs/op

See https://go.dev/ref/spec#Passing_arguments_to_..._parameters.

RELEASE NOTES: None

[arjan-bal]

LGTM, thanks for the contribution!

Adding another reviewer for a second approval.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.85%. Comparing base (005b092) to head (52792d2).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7525      +/-   ##
==========================================
- Coverage   81.91%   81.85%   -0.07%     
==========================================
  Files         361      361              
  Lines       27825    27825              
==========================================
- Hits        22794    22776      -18     
- Misses       3841     3853      +12     
- Partials     1190     1196       +6     

see 12 files with indirect coverage changes

[dfawley]

Wow, I wish otel had renamed WithAttributeSet to WithAttributes and made some other function behave the way WithAttributes behaves. That is unexpected. I might even go so far as to mark WithAttributes deprecated if I was them, to make it more clear that it's probably not what you ever want to use.