Make telemetry capture/writing thread-safe.

[anjagruenheid]

The current event stream for the telemetry registry is not synchronized. As a result, events might be added to the queue while it's being flushed in which case we'd not accurately report all executed events.

[jcamachor]

This issue should include tests for race conditions, e.g., writing while flushing, exceptions at different points while flushing, etc.