Expand trace context propagation docs

Scott Frenchrequested to merge
extend-context-propagation-docs into main

This expands the existing README.md section that introduces XRequestPropagator in a couple of key ways:

  1. It more affirmatively recommends use with W3CTraceContextPropagator, since use without that is likely to result in broken tracing.
  2. It makes explicit the dependency on Node SDK instrumentations.
  3. It calls out to the Wikitech docs, which will soon be updated with far more detail about WMF-production configuration.

Bug: T416756

Merge request reports