Need Of a replication lag Metric

[akhtar-a]

What happened
In a master slave configuration, where we have a master node and many followers nodes, a continuous asynchronous replication is happening between the the primary node to the secondary/replica node. There could be a chances where there is a signification lag in the replication.
What you expected to happen
In order to efficiently monitor the replication lag, there is a need to have a metric which accounts for the replication lag between the primary and the followers. Replication lag metic can be used for monitoring and alerting in case of the lag increase a certain threshold which will be useful in a cluster architecture.
How to reproduce it (as minimally and precisely as possible)
Create a primary-slave configuration.
Environment

# run "immu* version" and copy/paste the output here

Additional info (any other context about the problem)