Introduce event seen counter for snapshots

We want to consider adding a snapshot rate metric that is calculated based on the total number of events seen against the snapshot's duration in milliseconds.

We currently only capture the total number of events seen in the common metrics, but this counter is all events, including CREATE, UPDATE, and DELETE. This means that once the connector has transitioned to streaming and if incremental snapshots are started, this metric would be inaccurately calculated.

The SnapshotChangeEventSourceMetrics should expose a TotalNumberOfSnapshotEventsSeen metric that only tracks the READ events. This snapshot rate should then be calculated based on this event counter and the snapshot duration, which would give an accurate rate calculation for both initial snapshots and for incremental snapshots during the streaming phase.