Improve Prometheus stats + Add Grafana dashboard (#785)

We were missing some labels on metrics generated by the Prometheus exporter so I fixed that. There are still some gaps that I want to address with respect to the metrics we track but this seems like a good start.

I also created a Grafana Dashboard and exported it to JSON. It is designed with the same metric names the Prometheus exporter uses.

Author: drdrsh

README.md

@@ -268,6 +268,8 @@ psql -h 127.0.0.1 -p 6432 -d pgbouncer -c 'SHOW DATABASES'
 
 Additionally, Prometheus statistics are available at `/metrics` via HTTP.
 
+We also have a [basic Grafana dashboard](https://github.com/postgresml/pgcat/blob/main/grafana_dashboard.json) based on Prometheus metrics that you can import into Grafana and build on it or use it for monitoring.
+
 ### Live configuration reloading
 
 The config can be reloaded by sending a `kill -s SIGHUP` to the process or by querying `RELOAD` to the admin database. All settings except the `host` and `port` can be reloaded without restarting the pooler, including sharding and replicas configurations.