New metrics and thresholds for streaming delta

[ioguix]

Hi,

Since v10, pg_stat_replication includes three new metrics: write_lag, flush_lag and replay_lag.

At the very least, these three metrics should be included in perfdata.

Moreover, we could adapt the current threshold format to label the field the thresholds apply on, the way we do for eg. backends_status. this would support: write_lsn, flush_lsn, replay_lsn, write_lag, flush_lag, replay_lag.

Also, we should keep compatibility with current format. Here are some exemples:

Old format	New format
-w 2GB	-w flush_lsn=2GB,replay_lsn=2GB
-w 2GB,1GB	-w flush_lsn=2GB,replay_lsn=1GB
N/A	-w flush_lsn=2GB,flush_lag=1m
N/A	-w replay_lsn=2GB,replay_lag=1m

[Krysztophe]

👍

[ioguix]

This future work would also answer the feature request from PR #206