Track how many new messages are discovery after each periodic store query

[adklempner]

https://forum.vac.dev/t/receive-message-reliability-for-status-desktop/302

Ideally, the number of newly discovered messages is low (though it cannot be zero)

First version just tracks total messages missed
Second version of this metric should determine how many of the missed messages the client actually cared about

Note that these queries need to be distinct from recovery store queries

[adklempner]

relevant: status-im/status-go#5455