Admin page: add metrics endpoint

[jenshu]

• Decide what metrics to show
• Decide what library to use. Currently we call go-utils which uses opencensus which is no longer maintained. Consider migrating to otel, and implementing the metrics code in this repo instead of go-utils.
• Need to be able to scrape metrics from outside the pod (can use port 9090)

[yuval-k]

Need to be able to scrape metrics from outside the pod, so probably keep using the existing port 9091

i think after the recent change to the admin page 9091 became local only; so metrics can be in a different port, like 9090.

[krisztianfekete]

For metrics, I'd go with prometheus/client_golang, just like we do in GME. Far more stable, lightweight, performant than OTel, also much easier to work with and expose metrics locally.
It's also not a concern compatibility-wise, as we've been using it and consuming with Prometheus over a year now with customers in production without any issues related to the the mix.