Add a way to get events out of the JVM

This will need to be done individually for each SDK, but if they can also be 'agents' and give their perspective on request/response with latency, that'd be hugely useful to find higher level causes, such as a JVM GC.

No files, tests, or entry points are named. Start by clarifying how each SDK should expose JVM events and request/response latency, then define what evidence would identify higher-level causes such as JVM GC; done criteria are not specified in the issue.

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