data + custom tags + counters.

some extra work needed as pgpool SHOW commands don't specify the return data types for some reason.

connects actually to the instance to determine PG relevant disk paths / mounts.

assumes upwards compatibility for versions.

every DB under monitoring should have exactly 1 sql.DB connection assigned, that will internally limit parallel access.

LoadMetricDefs loads metric definitions from the reader.

SyncMetricDefs refreshes metric definitions at regular intervals.

Called once on daemon startup to try to create "metric fething helper" functions automatically.

Called once on daemon startup if some commonly wanted extension (most notably pg_stat_statements) is missing.

synchronize initial PG version detection to 1 instance for each defined host.