feat(flask): Add sentry_trace() template helper

[BYK]

To setup distributed tracing links between a Flask app and a front-end app, one needs to figure out how to get the current hub, safely get the traceparent and then properly pass it into a template and then finally use that properly in a meta tag. The guide is woefully inadequate and error-prone so this PR adds a built-in helper sentry_trace() to the Flask integration to simplify this linking.

I'll follow up with documentation updates once the approach here is approved.

[sl0thentr0py]

@BYK the tests on py3.4/old flask versions fail because there's no add_template_global apparently, maybe add a hasattr check and patch only if exists.

[sl0thentr0py]

hmm no ignore that, add_template_global exists since version 0.10, so something else going on here.

[BYK]

@sl0thentr0py I figured out the issue here. It's what @untitaker said. Flask.__call__() creates you a wsgi app from an already existing Flask instance. I got confused by that. I'll try to use before_render_template signal but not sure if I can modify the context at that point. Will try.

[BYK]

@silent1mezzo finally, ready to merge. Sorry for all that noise!

[BYK]

@untitaker @mitsuhiko @sl0thentr0py I don't think that format job will ever run

[BYK]

@sl0thentr0py it won't because this is set to run on push: https://github.com/getsentry/sentry-python/blob/master/.github/workflows/black.yml#L3

But I can only push to my own repo.

[sl0thentr0py]

yea we need to kill that black action and move to pre-commit, it's super annoying.
I can't merge without it passing because we removed admin overrides 2 weeks ago, sigh..

[BYK]

I can't merge without it passing because we removed admin overrides 2 weeks ago, sigh..

How do you release with Craft then?

[sl0thentr0py]

I just can't override required checks and merge now. I run the release workflow here and someone else approves. Dunno how/if those are related?

[BYK]

@sl0thentr0py see item 3 here: https://github.com/getsentry/publish#setup

Either you no longer have admin permissions, or you folks started using the "Allow specific actors to bypass pull request requirements" in branch protection rules (which means that README on publish needs to be updated), or you'll have trouble when you try to make the next release 😀

[AbhiPrasad]

We might want to do something similar for django as well. See: #1171

[BYK]

@AbhiPrasad if only Functional Software Inc. was still paying me... 😛

[AbhiPrasad]

if only Functional Software Inc. was still paying me

Well if ever get the itch to help out, opened #1341 to track. Thanks for your help BYK, glad to see you're still around :)