Add more pool metrics

[An-DJ]

Is your feature request related to a problem? Please describe.
There is a lack of metrics to characterize the pool state.

Describe the solution you'd like

• Add pgagroal_query_count which characterizes the number of queries in the most recent time interval.
• Add pgagroal_tx_count which characterizes the number of transactions in the most recent time interval.
• Add pgagroal_wait_time which characterizes the previous client waiting time.

[jesperpedersen]

Lets start with a simple guard of if (config->metrics > 0)