Update prometheus metrics list

[xtremerui]

1. starts up a concourse with the prometheus adapter
2. go to the metrics endpoint and copy the output
3. paste it into https://github.com/concourse/docs/blob/master/lit/docs/operation/metrics.lit#L46

next time the docs is built, it’ll parse that and make it human readable

Also remove the broken link of Concourse datadog metric URL as we switched to vmware wavefront.