feat: Add email send operation metrics

[jnschaeffer]

What kind of change does this PR introduce?

This PR adds two new metrics, gotrue_email_send_operations_counter_total and gotrue_email_send_errors_counter_total, for tracking email send operations and errors respectively. The purpose of these metrics is to track email delivery independently of the various API handlers that may send email within Auth.

What is the current behavior?

Users who want to track email sending operations in Auth must use indirect metrics like HTTP status codes to determine whether an email was successfully sent.

What is the new behavior?

The gotrue_email_send_operations_counter_total counter will increment each time Auth attempts to send an email after performing validation and checking rate limits. The gotrue_email_send_errors_counter_total counter will increment each time an email send attempt fails.

[coveralls]

Pull Request Test Coverage Report for Build 21297341599

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 7 (28.57%) changed or added relevant lines in 1 file are covered.
• 648 unchanged lines in 14 files lost coverage.
• Overall coverage increased (+0.04%) to 68.819%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/mail.go	2	7	28.57%

Files with Coverage Reduction	New Missed Lines	%
internal/api/samlacs.go	3	3.82%
internal/api/pkce.go	8	79.71%
internal/api/sso.go	14	70.18%
internal/models/flow_state.go	18	80.0%
internal/api/context.go	22	77.71%
internal/api/oauthserver/auth.go	26	0.0%
internal/api/oauthserver/service.go	29	77.06%
internal/models/oauth_client.go	31	73.23%
internal/api/middleware.go	39	76.65%
internal/api/verify.go	39	75.6%

Totals
Change from base Build 20599680500:	0.04%
Covered Lines:	14849
Relevant Lines:	21577

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💛 - Coveralls