[xCluster] Lag metrics remain at 0 if no GetChanges calls received

[rahuldesirazu]

Jira Link: [DB-343](https://yugabyte.atlassian.net/browse/DB-343)
In an xCluster replication setup, if the producer never receives a GetChanges call, the lag metric remains at 0. This may happen in the case of a network partition and may prevent us from seeing lag increase as the producer still takes writes.