Add simple metrics endpoint for rageshake.

[michaelkaye]

Import prometheus metrics library and expose default metrics on alternate port (if configured).

If no special configuration is made then no change should be visible.

Example startups:

^Cmichaelk@michaelk-XPS-13-9370:~/work/rageshake$ docker run -v /home/michaelk/work/rageshake/rageshake.yaml:/rageshake.yaml rageshake
No github_token configured. Reporting bugs to github is disabled.
No gitlab_token configured. Reporting bugs to gitlab is disaled.
No slack_webhook_url configured. Reporting bugs to slack is disabled.
No generic_webhook_urls configured.
2023/11/08 10:38:03 Using http://localhost:9110/api/listing as public URI
2023/11/08 10:38:03 Listening on :9110
^C

michaelk@michaelk-XPS-13-9370:~/work/rageshake$ docker run -p 9112 -v /home/michaelk/work/rageshake/rageshake.yaml:/rageshake.yaml rageshake -metrics :9112
No github_token configured. Reporting bugs to github is disabled.
No gitlab_token configured. Reporting bugs to gitlab is disaled.
No slack_webhook_url configured. Reporting bugs to slack is disabled.
No generic_webhook_urls configured.
2023/11/08 10:38:31 Using http://localhost:9110/api/listing as public URI
2023/11/08 10:38:31 Listening on :9110
2023/11/08 10:38:31 Metrics Listening on :9112
^C

When metrics listening they're available like so:

michaelk@michaelk-XPS-13-9370:~$ curl http://localhost:9112/metrics
# HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles.
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{quantile="0"} 0
go_gc_duration_seconds{quantile="0.25"} 0
go_gc_duration_seconds{quantile="0.5"} 0
go_gc_duration_seconds{quantile="0.75"} 0
go_gc_duration_seconds{quantile="1"} 0
...