chore(integrations): oauth2 identity pipeline metrics round 2

[cathteng]

Changes from #79325

Fixes SENTRY-3H4H

• We know that the slug must be some IdentityProvider. These are registered via IdentityManager, we can just attempt to get the IdentityProvider class from the manager with the key to check if the slug is valid. This will also handle registrations done in getsentry (e.g. google_login).

---

sentry/src/sentry/identity/__init__.py

Lines 25 to 35 in d7da2a2

	register(SlackIdentityProvider) # NOQA
	register(GitHubIdentityProvider) # NOQA
	register(GitHubEnterpriseIdentityProvider) # NOQA
	register(VSTSNewIdentityProvider) # NOQA
	register(VSTSIdentityProvider) # NOQA
	register(VstsExtensionIdentityProvider) # NOQA
	register(VercelIdentityProvider) # NOQA
	register(BitbucketIdentityProvider) # NOQA
	register(GitlabIdentityProvider) # NOQA
	register(GoogleIdentityProvider) # NOQA
	register(DiscordIdentityProvider) # NOQA

These are used in the installation flows for:

• GitHub Enterprise
• VSTSNewIdentityProvider
• Slack
• Vercel
• Gitlab
• GitHub (via a redirect, the pipeline is not directly used)

Missing:

• VSTSOAuth2CallbackView overwrites the exchange_token function (VSTSIdentityProvider, VSTSExtensionIdentityProvider)

Not applicable

• Bitbucket (already done separately in chore(bitbucket): installation step metrics #78980)
• Discord (does not subclass OAuth2Provider but is still registered as an IdentityProvider)
• Google (not attached to an integration)

[codecov]

Codecov Report

Attention: Patch coverage is 78.68852% with 13 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/identity/oauth2.py	77.58%	10 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #80216      +/-   ##
==========================================
+ Coverage   77.16%   78.08%   +0.92%     
==========================================
  Files        7179     7179              
  Lines      317140   317163      +23     
  Branches    43725    43725              
==========================================
+ Hits       244718   247661    +2943     
+ Misses      65878    63152    -2726     
+ Partials     6544     6350     -194