[opentracer] Documentation

[Kyle-Verhoog]

Add some basic documentation for our OpenTracing implementation.

[palazzem]

Good as a start. But I guess the next big task should be rewrite entirely our documentation. We should have the same quality as other Tracers, for example Java is at the moment the best docs we have.

For now, let's update the snippet so we can merge it.

[palazzem]

keep only:

config = {
    'agent_hostname': 'localhost',
    'agent_port': 8126,
}

because are the most common configs developers are changing.

Also scope_manager=ThreadLocalScopeManager is not mandatory so let's remove it. I'd say to describe the interface, so that people knows how to set the scope_manager, plus link to OT docs / repo for available scope managers. I prefer a snippet that is a kind of "copy-paste ready" for common cases. Also I've noticed that here we expect the instance and not the class so in any case it should be ThreadLocalScopeManager() right?

[palazzem]

Now that I think about it, would be great if you can put some examples directly here like we do for Java docs: https://github.com/DataDog/documentation/blob/bde053b60376d517961c46047d2be0a0254cf75c/content/tracing/setup/java.md#opentracing-api

You can use the same examples, translated in Python. When we'll refactor the docs, we'll move that section in the right place.

[palazzem]

we should check what are the choices made here: opentracing/opentracing-python#95