Feature request: Expose HTTP request metrics

[juliusv]

As far as I can see, kamal-proxy does not yet expose metrics about the requests it serves. It would be really nice if it tracked e.g. Prometheus metrics for things like:

• Request counts (including status code label dimension)
• Request duration histogram
• Request size histogram
• Response size histogram
• Current number of in-flight requests
• Etc.

[northeastprince]

Request metrics seen more like a responsibility for the apps themselves.

[juliusv]

That's ok, though it's pretty common to have them in both places. Usually apps are instrumented, and proxies / LBs like nginx, HAProxy, etc. also track metrics about requests going to their services and backends.