ref(hybridcloud): separate out explicit outbox model scheduling

[cvxluo]

Just moving this func around to support #121985. For the GAL, we'll want to run separate recovery tasks.

[sentry]

Bug: The outbox_name metric tag is now lowercase (e.g., sentry.controloutbox), which is a breaking change from the previous capitalized version, affecting existing monitoring and alerts.
_{Severity: HIGH}

Suggested Fix

To maintain backward compatibility for metrics, ensure the outbox_name tag value remains capitalized. Revert to using the outbox_name variable that was previously used, which is derived from the settings and has the correct casing.

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/hybridcloud/tasks/deliver_from_outbox.py#L114

Potential issue: The `outbox_name` tag for metrics was previously derived from
`settings.SENTRY_OUTBOX_MODELS`, resulting in capitalized values like
`"sentry.ControlOutbox"`. The new implementation at this location uses
`outbox_model._meta.label`, which produces lowercase values like
`"sentry.controloutbox"`. This case change is a breaking change for any monitoring,
dashboards, or alerts that filter on the `outbox_name` tag. The change will not raise an
exception but will silently cause existing monitoring queries to fail, breaking
historical continuity and potentially delaying responses to production issues.

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