Instrument Concurrent::RubyExecutorService#post

[kaylareopelle]

Acceptance Criteria

• Create a segment when Concurrent::RubyExecutorService#post is called, and add that segment to the existing transaction
  • This method does not need to create a transaction because it is most likely taking place within a library we already instrument to start transactions
• Allow customer to rename segment to something more specific

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-5128

[kaylareopelle]

Blocked by: #1602

[kaylareopelle]

#1602 is complete! All clear. Please branch off concurrent_ruby

[kaylareopelle]

Changing the focus for this ticket to the RubyExecutorService class, at a minimum, instead of the ExecutorService class. The post method in ExecutorService is a no-op. Previously, we thought it could be an access point for the classes that inherit from it.

We'll need to add instrumentation for each ExecutorService subclass individually.
Concurrent::Promises#future and Concurrent::Future#execute both route through RubyExecutorService#post.

[kaylareopelle]

Here are the caller locations for three methods we instrumented, showing they all route through Concurrent::ThreadPoolExecutor#post, by way of RubyThreadPoolExecutor#post.

Caller locations for Concurrent APIs.txt