Improve Prometheus exporter

[lenguyenthanh]

Follow up: lichess-org/lila#16500,

need to update applicaton.conf:

-    prometheus-reporter.enabled = yes
+    prometheus-reporter {
+      enabled = yes
+      factory = "lila.ws.PrometheusReporter$Factory"
+    }

tested locally

[ornicar]

shouldn't that go to a scalalib subproject instead?

[lenguyenthanh]

I don't think this is something we should maintain in the long term. It's (hopefully) a temporary single file, which can be removed after kamon fixing the issue.

Moving this to scalalib is a bit overkill unnecessary , I think.

[ornicar]

we'll see how well that comment ages.