feat(notifications): Pre compute notification fields prior to calling into the platform

[Christinarlong]

Summary

• Hooks SlackMetricAlertHandler into the Notification Platform (NP) replacing the send_incident_alert_notification() call
• Pre-computes attachment info and chart URL in the handler and placing those into the data (reducing lookups)
• Eliminates Activity vs GroupEvent branching in the renderer — invoke_legacy_registry already resolves both paths into the same context objects before send_alert() is called

Note:

• We're no longer hooking in at the noa layer atp (diff from original spec), so we didn;t really get rid of the invoke_legacy_registry part which may have been a goal?

[sentry-warden]

Detector.objects.get() can raise DoesNotExist without handler (src/sentry/notifications/notification_action/metric_alert_registry/handlers/slack_metric_alert_handler.py:142)

Line 142 uses Detector.objects.get(id=alert_context.action_identifier_id) without a try/except for Detector.DoesNotExist. If the detector is deleted between when the invocation is created and when send_alert runs, this will raise an unhandled exception. The check on line 143-144 if not detector is dead code since .get() either returns an object or throws - it never returns None.

Identified by Warden sentry-backend-bugs

[sentry]

Bug: Removing ACTIVITY_METRIC_ALERT from NotificationSource will cause deserialization to fail for existing NotificationThread records with that key_type, raising an error.
_{Severity: MEDIUM}

Suggested Fix

To prevent deserialization failures, either add a data migration to update or remove existing NotificationThread records with key_type="activity-metric-alert", or implement a fallback mechanism to handle the legacy source type gracefully during deserialization, perhaps by mapping it to the new MetricAlertNotificationData path or ignoring it.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/sentry/notifications/platform/types.py#L61

Potential issue: The pull request removes `ACTIVITY_METRIC_ALERT` from the
`NotificationSource` enum and its corresponding template from the registry. If any
`NotificationThread` records with `key_type="activity-metric-alert"` exist in the
database, any operation that requires deserializing them (such as replying to an old
notification) will fail. The `deserialize_notification_data` function will attempt to
look up the template for the source `"activity-metric-alert"`, which no longer exists in
the registry, causing a `NoRegistrationExistsError`. This will lead to a crash when
interacting with older metric alert notification threads.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[Christinarlong]

ACTIVITY_METRIC_ALERT shouldnt be used anywehre anymore

[GabeVillalobos]

Changes look good! Thanks for overhauling this.

[GabeVillalobos]

Are we still planning to hoist this into the NOA layer? Seems like we don't need the full open_period_context anymore at the very least.

[Christinarlong]

sadly we can't hoist this up as get_started_at turns the text into a slack sepcific format

[getsentry-bot]

PR reverted: 36ffdae