feat: log external latency

[zepatrik]

With this change, we measure and log some external latency separately.

Right now, we consider this external:

• OAuth2 provider APIs
• webhooks

Other calls, like loading jsonnet or courier templates, are not considered external because they are tightly coupled to the server Kratos is running on.

[codecov]

Codecov Report

Attention: Patch coverage is 61.29032% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 78.08%. Comparing base (cc39f8d) to head (d5d5a49).
Report is 2 commits behind head on master.

Files	Patch %	Lines
selfservice/strategy/oidc/provider_lark.go	0.00%	5 Missing ⚠️
selfservice/strategy/oidc/provider_patreon.go	0.00%	4 Missing ⚠️
selfservice/strategy/oidc/provider_dingtalk.go	40.00%	3 Missing ⚠️
selfservice/strategy/oidc/provider_spotify.go	0.00%	3 Missing ⚠️
schema/handler.go	50.00%	1 Missing and 1 partial ⚠️
selfservice/strategy/oidc/provider_github.go	0.00%	2 Missing ⚠️
selfservice/strategy/oidc/provider_github_app.go	0.00%	2 Missing ⚠️
selfservice/strategy/oidc/provider_vk.go	66.66%	1 Missing ⚠️
selfservice/strategy/oidc/provider_x.go	0.00%	1 Missing ⚠️
selfservice/strategy/oidc/provider_yandex.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3868      +/-   ##
==========================================
+ Coverage   78.04%   78.08%   +0.04%     
==========================================
  Files         360      361       +1     
  Lines       25276    25274       -2     
==========================================
+ Hits        19726    19736      +10     
+ Misses       4038     4025      -13     
- Partials     1512     1513       +1     

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[alnr]

Looks good, just one comment.