create a prometheus exporter on a standard http port (#107)

* create a hyper server and add option to enable it in config

* move prometheus stuff to its own file; update format

* create metric type and help lookup table

* finish the metric help type map

* switch to a boolean and a standard port

* dont emit unimplemented metrics

* fail if curl returns a non 200

* resolve conflicts

* move log out of config.show and into main

* terminating new line

* upgrade curl

* include unimplemented stats

docker-compose.yml

@@ -14,3 +14,4 @@ services:
       - "${PWD}/examples/docker/pgcat.toml:/etc/pgcat/pgcat.toml"
     ports:
       - "6432:6432"
+      - "9090:9090"