Add a benchmark for aspnetcore

[kevingosse]

The benchmark goes through a typical customer scenario: receive a request through aspnetcore, and query an external HTTP service.

[kevingosse]

/azp run Benchmarks

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[zacharycmontoya]

nit: I think you can set this in the TracerSettings below with TraceEnabled = false so you don't have to modify the environment variables

[kevingosse]

The problem is subtle:

to initialize the tracer, I do:

Tracer.Instance = new Tracer(settings, new DummyAgentWriter(), null, null, null);

By calling new Tracer, the static constructor is invoked, which initializes a new tracer before I have time to construct mine. That tracer is fully functional, with background threads and stuff, and adds noise to the benchmark. Adding environment variables is the only way I found to limit what that tracer does in background.
Now that I think about it, I should probably just add a global internal setting that prevents that tracer from being instantiated. I'll look into that in a separate PR.

[lucaspimentel]

Could we move the creation of the first Tracer instance from the static constructor to the Tracer.Instance getter? If we can do this, it should allow users (including ourselves here) to set Tracer.Instance without creating a not-so-throwaway Tracer instance. Just something to think about, not for this PR.

[zacharycmontoya]

Looks good! Looking forward to having more benchmarks!