Add more client metrics

[An-DJ]

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

Describe the solution you'd like

• Add pgagroal_client_active which characterizes the number of active clients Add metrics of client #157
• Add pgagroal_client_wait which characterizes the number of waiting clients Add metrics of client #157
• Add per-db statistics to the metrics above

[jesperpedersen]

Maybe this is more related to #105

[jesperpedersen]

Completed tasks for GSoC 2021