Export last replay age in replication collector #1085
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The exported replication lag does not handle all failure modes, and can report 0 for replicas that are out of sync and incapable of recovery.
A proper replacement for that metric would require a different approach (see e.g. #1007), but for a lot of folks, simply exporting the age of the last replay can provide a pretty strong signal for something being amiss.
I think this solution might be preferable to #977, though the lag metric needs to be fixed or abandoned eventually.