Tracing support

[xmo-odoo]

This is mostly for the test suite as currently the experience is not great (especially when a problem chain involves dummy-central).

Rough idea:

• have a fixture generate a trace id (per test) if enabled
• do magic so capsys & co output their stuff to tracing (?)
• make env pass that to the Environment, update Environment to add the trace id as a header to xmlrpc (ServerProxy(header) support was added in 3.8)
• mangle logging to support tracing output (otel apparently has some integration with logging)
• similarly with make_repo and the github session
• update the mergebot/forwardbot to forward the trace id when it calls github (can just get it from the request headers)
• update DC to take up that trace id and add it to its tracing context
• find a way to collate all the tracing stuff back into pytest's stream capture things, so it's all displayed nice and tight in case of failure (see CaptureManager, possibly terminal_reporter, not entirely sure how to make pytest retrieve the tracing data, most likely all three processes need some config to sync on)