Add Rake instrumentation

[delner]

This pull request adds instrumentation for Rake. Each task will generate spans named either rake.invoke, rake.execute, or both, depending on how the task runs.

To enable this, you must add the following to your Rakefile:

# Rakefile
require 'ddtrace'

Datadog.configure do |c|
  c.use :rake, service_name: 'dd_test-rake'
end

# Define your rake tasks here...

Screenshot of a db:migrate task in action:

[delner]

This depends on #410: that PR should be merged before this one.

[palazzem]

@delner #410 merged and this one rebased on top of 0.13-dev. Reviewing it now!

[palazzem]

Great contribution! I think we only need to add documentation and it's ready to be merged!

[delner]

@palazzem Ah, right. Will do that.

[delner]

@palazzem Added some documentation, but discovered some issues with the quantization options. Added tests and a fix for that, and changed the tags a bit. Should be good for another review now.

[delner]

Flaky gRPC tests are preventing the test build from passing right now.