create a prometheus exporter on a standard http port

[dat2]

This creates a prometheus exporter in process. This was requested in #83.

If you add enable_prometheus_exporter = true to pgcat.toml then you can make a GET request to <pgcat host>:9930/metrics to get a list of metrics.

Here's an example of the prometheus metrics exporter endpoint.

Reference: https://prometheus.io/docs/instrumenting/writing_exporters/

➜  pgcat git:(nickdujay/metric-exports) ✗ http -v :9930/metrics
GET /metrics HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:9090
User-Agent: HTTPie/3.2.1



HTTP/1.1 200 OK
content-length: 7259
content-type: text/plain; version=0.0.4

# HELP pgcat_sv_idle How many server connections are idle
# TYPE pgcat_sv_idle gauge
pgcat_sv_idle{shard="0",host="127.0.0.1",database="simple_db",role="primary"} 1

# HELP pgcat_avg_recv Average of total_received every 15 seconds
# TYPE pgcat_avg_recv gauge
pgcat_avg_recv{role="primary",shard="0",host="127.0.0.1",database="simple_db"} 3

# HELP pgcat_total_received Number of bytes received from the server
# TYPE pgcat_total_received counter
pgcat_total_received{host="127.0.0.1",shard="0",database="simple_db",role="primary"} 54

# HELP pgcat_avg_wait_time Average of total_wait_time every 15 seconds
# TYPE pgcat_avg_wait_time gauge
pgcat_avg_wait_time{host="127.0.0.1",role="primary",shard="0",database="simple_db"} 1042
#truncated

[levkk]

Amazing, thank you so much!