Proposal: Metrics Support

[martin31821]

We've been running nexus in several workloads for a long time now and the missing observability always drove us crazy.
Therefore I'm proposing to add metrics support to nexus.

This would utilize the prometheus client library and expose the following metrics:

• Bytes in/out
• Messages in/out
• Publications per URI
• Calls per URI, track runtime into buckets
• Subscribe/Unsubscribes per URI
• Register/Unregister per URI
• Connects/Disconnects
• a grafana dashboard / set of promQL queries to show how to use these metrics
• add a /metrics endpoint to the HTTP server to get the metrics.

If you'd be willing to accept a PR, @johannwagner and I would go ahead and implement it.