Add table sizes

[mowirth]

This MR adds total_relation_size, relation_size, indexes_size for all tables in all schemas.

Implements #1036

[kostya-skw]

I'm waiting for it

[bilkoua]

@sysadmind Would it be possible to add this functionality?

[azdfzshffg]

Up ! Would it be possible to add this functionality?

[azdfzshffg]

@sysadmind @spantaleev it be possible to add this functionality?

[bilkoua]

[inputs.sql]
driver = "postgres"
dsn = "host=/run/postgresql user=myuser dbname=mydb"
[[inputs.sql.query]]
query = "SELECT current_database() AS db, schemaname, relname, pg_total_relation_size(relid) AS total_size_bytes FROM pg_statio_user_tables"
tag_columns_include = ["db", "schemaname", "relname"]