Add metric for max postgresql transaction ID age

I've been reading a little about PostgreSQL transaction ID wraparound
today, and how it's knocked multiple companies out of commission for
days to get it resolved. It should have almost no chance of happening on
Tildes for years, but this will let me set up some monitoring for it
now, while I'm thinking about it.

For more info:
https://blog.sentry.io/2015/07/23/transaction-id-wraparound-in-postgres.html

salt/salt/prometheus/exporters/postgres_exporter.sls

@@ -16,6 +16,13 @@ unpack-postgres-exporter:
     - options: --strip-components=1
     - enforce_toplevel: False
 
+/opt/prometheus_postgres_exporter/queries.yaml:
+  file.managed:
+    - source: salt://prometheus/exporters/queries.yaml
+    - user: postgres
+    - group: postgres
+    - mode: 644
+
 /etc/systemd/system/prometheus_postgres_exporter.service:
   file.managed:
     - source: salt://prometheus/exporters/prometheus_postgres_exporter.service

salt/salt/prometheus/exporters/prometheus_postgres_exporter.service

@@ -9,6 +9,7 @@ WorkingDirectory=/opt/prometheus_postgres_exporter
 User=postgres
 Group=postgres
 Environment="DATA_SOURCE_NAME=user=postgres host=/run/postgresql/ sslmode=disable"
+Environment="PG_EXPORTER_EXTEND_QUERY_PATH=/opt/prometheus_postgres_exporter/queries.yaml"
 ExecStart=/opt/prometheus_postgres_exporter/postgres_exporter
 
 [Install]

salt/salt/prometheus/exporters/queries.yaml

@@ -0,0 +1,6 @@
+pg_txid:
+  query: "SELECT max(age(datfrozenxid)) AS max_txid_age from pg_database"
+  metrics:
+    - max_txid_age:
+        usage: "GAUGE"
+        description: "Highest transaction ID age (wraparound at 2 billion)"