Add support for optional namespace caching. (#319)

In the user queries.yml file, the created namespaces can now be optionally
cached by setting cache_seconds, which will prevent the query being re-run
within that timeframe if previous results are available.

Supercedes #211, credit to @SamSaffron for the original PR.

queries.yaml

@@ -114,7 +114,8 @@ pg_statio_user_tables:
         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 FROM pg_database" 
+  query: "SELECT pg_database.datname, pg_database_size(pg_database.datname) as size FROM pg_database"
+  cache_seconds: 30
   metrics:
     - datname:
         usage: "LABEL"