Implement External Metrics/Tracing

The motivation for this task is that the wrappers will need a way to support external metrics/tracing without blocking during interpreter/VM transitions. This should not require much pathfinding at this point after our discussion.

Key notes:

• global lock problem in Node.js, Python, Ruby, PHP
• strategy is to queue and defer invocation of custom tracer API until next time we have the lock (so we're not blocking trying to obtain the lock each time)
• but need to document a limitation because this strategy can only be used with tracers that allow explicit timestamps
• Note that this is not for Metrics/Tracing in transactions itself but only in the main CB++ implementation