Async trace reporter readme

[chingor13]

Re-adding the example to the Trace README.md about using the AsyncReporter

[dwsupplee]

This looks great! A quick note, though - should we also notate that the AsyncReporter isn't 100% reliant on the batch daemon and that users can rely on register_shutdown_function or flush to process queued traces?

[chingor13]

I'm actually wondering if I should add a factory method on the TraceClient like we do for the pubsub batch publisher. The constructor for the AsyncReporter vs SyncReporter take different options and we can actually use AsyncReporter in place of SyncReporter (if IS_BATCH_DAEMON_RUNNING is false)

[dwsupplee]

That sounds like it would be a solid addition 👍.