chore(reports): add metrics to report schedule and log prune

[villebro]

SUMMARY

We've recently run into an issue with Celery worker processes becoming idle that's likely caused by an open regression in Celery 5 (see celery/celery#7276 for more details). While downgrading to Celery 4 isn't an option, one way of ensuring Alerts & Reports are running is monitoring the scheduling tasks. This PR adds a StatsD metric to reports.scheduler, and while at it, also does the same for reports.execute and reports.prune_log.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API