Trace sampler interface is now just a Proc

[dazuma]

Overall, clean up interfaces and documentation around sampling. Specifically:

• Change the method on the sampler interface from check to call so a Proc can be used. Sorry for that annoyance; I've been writing too much Java these past couple years.
• Make it explicit that the rack env is passed to the sampler.
• Path blacklist is really just a special case of sampling, so move that out of Middleware into TimeSampler. That lets us simplify the middleware interface a bit.
• Rename sampling.rb to time_sampler.rb because it defines TimeSampler.
• Clarify documentation around sampling, by including a section in the main Google::Cloud::Trace guide and adding details to the TimeSampler docs.

[blowmage]

I like this change. 👍

[dazuma]

@hxiong388 Updated docs to call out sampling and blacklisting. PTAL.