fix(monitoring): improve accuracy of replica lag

[gbartolini]

Synchronize the metrics' configmap with the current one shipped by the operator, while fixing the pg_replication metric's lag issue.

Closes #143

[felixscheinost]

How is the status on this?

[itay-grudev]

@gbartolini Why does this MR remove certain metrics? They are actively used by the Grafana Dashboard and by other users.

For example, the entire backends section is deleted.