Update github sync metadata timestamp to represent actual sync time

The community metrics data ingestion scripts run in a loop and import all new data since the last run. We have a [Source Data Freshness](http://104.154.241.245/d/data-freshness/source-data-freshness) dashboard which tracks the freshness of import data. A Jenkins job checks this data regularly to ensure our dashboards don't get stale.

Currently the GitHub "last sync" time actually tracks the last activity found on import. This means that if there is no activity on GitHub our dashboards will appear stale and our prober job will fail. This can happen in normal scenarios (i.e. at night, or during weekends / holidays).

We should update the last sync timestamp logic to be the actual timestamp of last poll, rather than the last data we found.

Imported from Jira [BEAM-5924](https://issues.apache.org/jira/browse/BEAM-5924). Original Jira may contain additional context.
Reported by: swegner.
Subtask of issue #19110