Allow caching of query results

[SamSaffron]

Getting pg database size is very expensive, especially if we need to do this across hundreds of dbs.

I would like an optional cache property per query that allows us to "remember" the last execution of the query for N seconds... Eg:

pg_database:
  query: " SELECT pg_database.datname, pg_database_size(pg_database.datname) as size FROM pg_database" 
  cache: 3600
  metrics:
    - datname:
        usage: "LABEL"
        description: "Name of the database"
    - size:
        usage: "GAUGE"
        description: "Disk space used by the database"

Is there any objection to adding such an optional attribute.

Currently we have no way of reporting DB size cause if we report it we hammer our DBs into oblivion

[baryluk]

+1, however I also think pg_database_size should be included in default set of metrics anyway ;) with suitable caching.

[wrouesnel]

Resolved by #319 which was a rewrite of #211 since quite a lot had changed. Hopefully working v0.7.0 which I just pushed to get these features out (latest will have them too).