Stabilize observability

[Liam-DeVoe]

Closes #4387.

• Disable coverage by default
  • IMO this is too much data (and too high risk for collision with sys.monitoring tools / slowdowns, but that's a weaker argument) to enable by default, but I'm open to discussion
• @settings(observability=...) takes three possible values:
  • observability=True enables observability
  • observability=False disables observability
  • observability=[option1, option2], like observability=["coverage"], enables observability, and also enables whatever options are specified
• if @settings(observability=...) is not set, inherit from HYPOTHESIS_OBSERVABILITY envvar
• HYPOTHESIS_EXPERIMENTAL_OBSERVABILITY_NOCOVER removed with no deprecation period
• HYPOTHESIS_EXPERIMENTAL_OBSERVABILITY and HYPOTHESIS_EXPERIMENTAL_OBSERVABILITY_CHOICES kept for backwards-compat because it was easy, but are now-undocumented and will be removed at some point

I'd like to keep a very clear separation between @settings(observability=...), and add_observability_callback. IMO the former should always write to/be related to .hypothesis/observed, and the latter should never be (ie it's sent in-memory).

This brings up a non-trivial issue, probably for a future PR: when adding observability callbacks, there's no way to control the coverage and choices options. You don't want to set @settings(observability=["choices"]) on the test/profile, because this would write to .hypothesis/observed, and you just want the observations in-memory. Previously this was done with observability.OBSERVABILITY_COLLECT_COVERAGE and observability.OBSERVABILITY_CHOICES. I'd like to get rid of those vars eventually, and add add_observability_callback(options=["coverage"]). But semantic questions like "what if two callbacks where only one sets options=["coverage"] have to be answered. I do think this can be done, I just don't want to do it right now. So I've left those vars in, though now un-documented.

[cc @hgoldstein95]

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I'd also love to write a hypothesis.works blog post about this, at some point..