uninstruemnt existing instances before uninstrumenting fastapi class

[TheAnshul756]

Description

Create a static list of all instances of InstrumentedFastAPI and fix uninstrument function.

Fixes #1256

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• tox -e test-instrumentation-fastapi

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added

[lzchen]

@TheAnshul756 @srikanthccv

So, as discussed, we wanted to uninstrument existing instrumented apps when we call instrumentor.uninstrument()

When was this discussed? As a user this makes sense to me but we'll have to do this for other instrumentations as well. Could we make tracking issues for those?

[srikanthccv]

This was briefly discussed in one of the SIG meetings when the topic was brought up by I believe Sanket. Some instrumentations haven't been fixed for this case but generally, we are trying to ensure the uninstrument works as expected. Sure, I will create an issue for the instrumentation we have missed this.

[sanketmehta28]

Resolve the generate test issue