Feature request: expose NoopTracer

[skaji]

Context

I'm using dd-trace-go in both development and production environments.
While I'm using a "true" tracer in production environment,
I'm using a mocktracer in development environment:

if os.Getenv("ENV") == "production" {
	tracer.Start()
} else {
	mocktracer.Start()
}

// share the same code in both production and development environments
span := tracer.StartSpan("some-resource")
// do something
span.Finish() // actual tracing happens only in production environment

Recently I have noticed that the memory usage in development environment was increasing.
This is because mocktracers keep all finished spans in memory.

dd-trace-go/ddtrace/mocktracer/mockspan.go

Line 203 in 1efe3a4

s.tracer.addFinishedSpan(s)

Feature request

I would like "noop" tracers.
There already exists NoopTracer in the internal package, so how about exposing it?

dd-trace-go/ddtrace/internal/globaltracer.go

Line 44 in 1efe3a4

type NoopTracer struct{}

Or, it may be better to add an option to mocktracers to disable keeping finished spans.

[skaji]

... oh, I just should not have started mocktracers in development environment.

[gbbr]

Glad you figured it out. If there is anything we can do better in documentation to make this clearer, please let me know...