Dd-trace-rb: Add support for `concurrent-ruby` `Concurrent::Promises::Future`

[benrbrook]

The concurrent-ruby pattern for using Futures is now contained in Concurrent::Promises::Future, and Concurrent::Future has been deprecated (https://github.com/ruby-concurrency/concurrent-ruby#deprecated). It would be great if we could add support for the new style of Futures so that tracing context is propagated for Concurrent::Promises::Future as well (https://github.com/DataDog/dd-trace-rb/blob/master/lib/ddtrace/contrib/concurrent_ruby/patcher.rb#L23).

[marcotc]

Seems like a more than reasonable request, @benrbrook. I'll add it to our roadmap. Given how small the required changes seem to be, I expect that we'll be able to address it without much issue.

[lloeki]

@marcotc This is a bit trickier than expected because of additional indirection in the new Promises::ChainPromise#on_resolvable, but I got something running. I'll clean that up next week and push a PR.