feat: Alert metrics in PostHog

[anirudhpillai]

Problem

Since we can't sent counters from celery jobs, sending metrics to PostHog

Changes

Send alert metrics to PostHog

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

N/A

How did you test this code?

Tested locally

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: posthog/tasks/alerts/checks.py

Function	Unhandled Issue
check_alert_and_notify_atomically	TemplateDoesNotExist: email/alert_check_failed_to_evaluate.html posthog.tasks.alerts.checks.c... Event Count: 5.2k
check_alert	ValueError: sleep length must be non-negative pos... Event Count: 45

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[anirudhpillai]

lmk if someone has a better idea of what to use instead 😅