Adding a tracer provider

[lykkin]

There seems to be some movement on implementing things from spans up, so I thought I would start building from the tracer factory down.

This currently requires C++14 for its use of unique_ptrs. I know there was some talk about wanting to limit the standard to C++11, but not sure how hard this requirement is.

[lykkin]

I signed it

[reyang]

It'll be great to point to the specification.
As what we have today, the spec is using TracerFactory and getTracer.
Also it'll be helpful to clarify the thread-safety requirement.

[SaintDubious]

How Serious are we about the "header only" requirement specified here: https://github.com/open-telemetry/opentelemetry-cpp/blob/master/docs/requirements.md

[g-easy]

tracer-factory.h

Should this be tracer_factory.h with an underscore, to match e.g. string_view.h?

[lykkin]

tracer-factory.h

Should this be tracer_factory.h with an underscore, to match e.g. string_view.h?

Good call, will change.

[jmacd]

This looks good. One nit: open-telemetry/oteps#76 says that we'll standardize on the term "Provider" instead or "Factory".

[lykkin]

This looks good. One nit: open-telemetry/oteps#76 says that we'll standardize on the term "Provider" instead or "Factory".

Thanks, I'll update that.

What do you think about making the return value of GetTracerFactory a shared_ptr<TracerFactory>? Since users can construct factories outside the global scope and pass it in, there's some questionable ownership of that pointer at that point. I can either add that in as part of this PR or make a quick follow on PR after this gets merged. I'd prefer the latter, as I would need to make nostd::shared_ptr for the ABI compat restriction.

[pyohannes]

@reyang: I just rebased the branch.