Create a (temporary) analytics script to track statistics about the wb_changes table in statsd. It should at least track the timestamp of the oldest change in wb_changes table.

This isn't great, because the analytics sripts access the replicas and thus might be lagged which in turn might lead to sometimes somewhat confusing data. However, this is still better than not having this data at all. We'll replace it with proper tracking with Promethues as soon as T240685: MediaWiki Prometheus support is done.

Acceptance criteria:

• create analytics script to get data out of wb_changes -> done in https://gerrit.wikimedia.org/r/c/analytics/wmde/scripts/+/725416, changes attached to T291276
• create Grafana panel for that data -> done: https://grafana.wikimedia.org/d/hGFN2TH7z/edit-dispatching-via-jobs
• Add another metric that tracks the number of rows in the wb_changes table, so we know if DispatchChanges job is not getting submitted
• Add alert to number of rows metric
• Alert is documented on https://wikitech.wikimedia.org/wiki/WMDE/Wikidata/Alerts