Expose more individual pieces of context for tighter integration between tracing and logging

[thenorili]

Notes from discussion with Jane explaining a long-running desire of hers for eyre:

might we want to expose backtrace() as API like anyhow does so that folks can rehydrate the contents for structured logging?

Desire: tighter integration btwn tracing and logging,
grabbing the individual pieces of context to log them all separately,
when you're printing an error into spantrace and tracing, are you printing the trace
where it's captured and it's printed or just the trace where
it's captured? printing both gives lots of information, it's unclear
how to format this, maybe guess and check.