Add pg_database collector

Converts the pg_database metrics from queries.yaml to a built in collector. This is enabled by default because it is not likely to be a performance problem and likely very useful data.

Signed-off-by: Joe Adams <github@joeadams.io>

queries.yaml

@@ -146,18 +146,6 @@ pg_statio_user_tables:
         usage: "COUNTER"
         description: "Number of buffer hits in this table's TOAST table indexes (if any)"
 
-pg_database:
-  query: "SELECT pg_database.datname, pg_database_size(pg_database.datname) as size_bytes FROM pg_database"
-  master: true
-  cache_seconds: 30
-  metrics:
-    - datname:
-        usage: "LABEL"
-        description: "Name of the database"
-    - size_bytes:
-        usage: "GAUGE"
-        description: "Disk space used by the database"
-
 # WARNING: This set of metrics can be very expensive on a busy server as every unique query executed will create an additional time series
 pg_stat_statements:
   query: "SELECT t2.rolname, t3.datname, queryid, calls, total_time / 1000 as total_time_seconds, min_time / 1000 as min_time_seconds, max_time / 1000 as max_time_seconds, mean_time / 1000 as mean_time_seconds, stddev_time / 1000 as stddev_time_seconds, rows, shared_blks_hit, shared_blks_read, shared_blks_dirtied, shared_blks_written, local_blks_hit, local_blks_read, local_blks_dirtied, local_blks_written, temp_blks_read, temp_blks_written, blk_read_time / 1000 as blk_read_time_seconds, blk_write_time / 1000 as blk_write_time_seconds FROM pg_stat_statements t1 JOIN pg_roles t2 ON (t1.userid=t2.oid) JOIN pg_database t3 ON (t1.dbid=t3.oid) WHERE t2.rolname != 'rdsadmin'"