Clarify lifecycle of snapshot metrics

As reported by sergeimorozov: when the connector is doing a snapshot and then transitions over to the streaming phase, the snapshot metrics will continue to be exported and thus for instance be visible in a dashboard. Now, when restarting the connector later on, and it starts directly in the streaming lifecycle phase, no snapshot metrics are exposed. This may be confusing for users.

Options to address this include:

• Persist snapshot metrics (for instance in the connector offsets), allowing to read them back after a restart; pro: full metrics at all times, con: increases offset size potentially a lot
• Derive a minimal snapshot metrics state after a restart; typically, this will only be something like "snapshot completed: true"; pro: no impact on offsets, showing some snapshot metrics at least; con: may be even more confusing due to the incomplete metrics state that's exposed
• Leave things as is, but clarify the lifecycle of snapshot metrics in the connector docs, e.g. here for MySQL