Additional Default Metrics about libzbxpgsql HOT 4 OPEN

@robbrucks I'm building these out now and reading up on how XIDs work.

Can you tell me the different between your first two queries, as I understand (select setting from pg_settings where name = 'autovacuum_freeze_max_age') should always resolve to 2000000000 anyway and is the runtime upper bound for AGE(datfrozenxid)?

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Thinking this through further... Instead of percentages, would it useful to have a Zabbix items with the raw XID age value and trigger for when the XID age comes within a precise range of the wrap limit? The triggers could use the same thresholds hard coded by Postgres; 10M and 1M as per https://github.com/postgres/postgres/blob/master/src/backend/access/transam/varsup.c#L267.

I find percentages are useful for visuals, but lack precision for triggers. Or should we build both?

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In 038cbe2 I've added a new key pg.db.xid_age which returns the age (by transaction count) of the oldest database xid or the given database xid. These are presented as items in the PostgreSQL MVCC application in the template.

I've also added triggers to the template to match the hard coded thresholds in PostgreSQL; 10M and 1M remaining xids.

Thoughts?

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That will work.

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