- Only continue tracing in TaskManager if a parent APM trace context exists. Trace headers might
be set by external transactions as well. If present, it caused APMTracer to report a transaction
for every span when in fact being sampled out.

- Correctly report the duration of transactions if not recording by not discarding the root span
immediately. These transactions might still get reported, but without spans.

- Discard transient trace start time in `newTraceContext` when a parent APM trace context
already exists. If propagated, all spans would start at the same time.

Relates to ES-13389