In-progress check-ins not being sent intermittently with Celery integration

### How do you use Sentry?

Sentry Saas (sentry.io)

### Version

2.29.1

### Steps to Reproduce

We've recently shipped an improvement to the Crons' monitor details page, and noticed that many in-progress check-ins are never being sent across all of our monitors. To reproduce, just set up our automatic detection integration with Celery and go to the monitor details page.

### Expected Result

All check-ins are sending the in-progress check-ins, and not only the completion check-in.

### Actual Result

In-progress check-ins are being missed:


Here's some of the impacted monitors:
- [Internal 🔗 ] [monitors-detect-broken-monitor-envs](https://sentry.sentry.io/issues/alerts/rules/crons/sentry/monitors-detect-broken-monitor-envs/details/?statsPeriod=7d)
- [Internal 🔗 ] [fetch-release-registry-data](https://sentry.sentry.io/issues/alerts/rules/crons/sentry/fetch-release-registry-data/details/?statsPeriod=1h)

Start with the automatic detection integration for Celery and trace how in-progress and completion check-ins reach the monitor details page. Compare the two check-in paths and verify that every in-progress check-in is sent for affected monitors, not only the completion check-in.

Written by the indexing model from the issue text.