Ability to force a transaction trace to be recorded

[itsderek23]

When debugging a production issue, it'd be helpful to force recording a transaction trace. This could be triggered by some custom logic that makes sense for the application, similar to the logic suggested in the server timing gem.

This logic could be leveraged by In-browser dev tools to ensure a full trace of server-side behavior is available as well. See w3c/server-timing#39 (comment).