Add recommendations for `process.pid` and `service.instance.id` in Python zero code

[tammy-baylis-swi]

For all options besides instrumenting post-fork, there is an issue where the Resources for each worker will be the same. For traces and logs it's OK, but metrics will have single writer violations.

We don't need to fix everything in this PR but maybe we can

• recommend instrumenting post-fork as the best option and
• recommend enabling the ProcessResourceDetector so that process.pid is detected correctly
• and ideally that each worker has a unique service.instance.id

WDYT?

Originally posted by @aabmass in #7081 (comment)

I'd like to add the recommendations around process.pid and service.instance.id in a new PR, if that's ok!