Implement Flask instrumentation support

[unflxw]

A Flask OpenTelemetry instrumentation exists, and supporting it would be neat. If Django is Rails, then Flask is Sinatra: it's the "light-weight", DIY option for making a web application.

To do

• Create a test setup for the instrumentation Add Python flask test-setup test-setups#112
• Write an extractor for the agent https://github.com/appsignal/appsignal-agent/pull/990
• Add an instrumentation adder for the package Add Flask integration #60
• Document it for the package setup in PR https://github.com/appsignal/appsignal-docs/pull/1297/commits/6e90668a64690ba8a295d9bf28a45aacda95d4a6

[unflxw]

@luismiramirez I notice now that the to do list says "document this for the existing stand-alone-based setup". We can still do that if we think it's valuable, but now that the future Python package docs are ready, we might want to only write the docs for the Python package and add them to that PR.

(We also haven't really tested this with the stand-alone setup, I don't think -- which is also fine in my view)

[tombruijn]

I suggest we write it for the new docs.

[luismiramirez]

Yep, my idea is to add the instructions to this PR