Update google-cloud-logging to use trace context tracking from core

[dazuma]

This PR includes two related changes in google-cloud-logging.

• The middleware uses stackdriver-core to obtain trace context instead of attempting to read it itself. As a result, stackdriver-core is now a gem dependency (as well as a dependency of gcloud, google-cloud, and stackdriver)
• The middleware allows setting the log name by path (so that, e.g. health checks in App Engine Flex can be directed to a separate log and don't spam the main app log.)

This PR is being directed to a feature branch for now because it is dependent on the release of the stackdriver-core gem.

[dazuma]

CircleCI failure seems to be unrelated:

  1) Error:
Google::Cloud.speech#test_0001_:
RuntimeError: This code example is not yet mocked
    /home/ubuntu/google-cloud-ruby/google-cloud-speech/support/doctest_helper.rb:69:in `new'
    /home/ubuntu/google-cloud-ruby/google-cloud-speech/lib/google-cloud-speech.rb:105:in `speech'
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:77:in `context'
    /home/ubuntu/google-cloud-ruby/google-cloud-speech/lib/google-cloud-speech.rb:102
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:56:in `eval'
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:56:in `evaluate_example'
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:43:in `block (4 levels) in generate'
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:40:in `each'
    /opt/circleci/.rvm/gems/ruby-2.3.1/gems/yard-doctest-0.1.8/lib/yard/doctest/example.rb:40:in `block (3 levels) in generate'

[blowmage]

The rebuild passed, so its probably nothing to worry too hard about for now.

I can't recreate the failure locally. Its possible that it was a fluke or that there is a way of sorting the speech doctest tests that will cause it to fail. @quartzmo this is something to keep in mind.