trace_events: background thread metadata events
V8 uses a thread pool provided by the host to schedule background tasks for concurrent GC and compiation. Emit trace events to identify the background threads. Ensure that the tracing infrastructure is started before the thread pool is initialized.
/cc @nodejs/trace-events @nodejs/diagnostics
EDIT: CI: https://ci.nodejs.org/job/node-test-pull-request/15001/