Health over Prometheus metrics

[sorki]

This patch adds /metrics route which exports metrics in Prometheus format (using encoding from prometheus library).

Sample output:

$ curl localhost:1442/metrics
# TYPE kupo_connected gauge
kupo_connected  1.0
# TYPE kupo_most_recent_checkpoint counter
kupo_most_recent_checkpoint  294998
# TYPE kupo_most_recent_node_tip counter
kupo_most_recent_node_tip  71753381

[sorki]

In the same spirit as Ogmios metrics PR.

[KtorZ]

Merged it as part of: be375a2

I did change a few things, namely:

• connected is renamed to connection_status
• the /metrics endpoint has been merged into /health and are distinguished through Accept headers. Accept: text/plain returns prometheus metrics, whereas Accept: application/json returns health as JSON. It defaults to JSON when no Accept is provided.