(feature) Support for metrics using Prometheus in relay proxy about go-feature-flag

[thomaspoignant]

Hello @Rongtao-Fu-Imprivata, this is actually a super nice idea and we should consider this for sure.
I think this is definitely a good way to improve the relay-proxy.

Let me try to find time to work on this in the near future or if you want you can contribute to the relay-proxy by opening a PR for that.

We could to the echo middleware https://echo.labstack.com/middleware/prometheus/

[thomaspoignant]

Also @Rongtao-Fu-Imprivata I would be interested to know which metrics you would be interested in.
Are API calls enough or do you want more than that, such as a refresh of the configs etc ... ?

[Rongtao-Fu-Imprivata]

Evaluation API call metrics like response time, size in bytes, success/failure rate are super helpful to monitor the service performance, throughput, reliability. Other interesting metrics can be number of evaluation rules, how often those rules are added/removed/updated, response time, success/failure rate to retrieve evaluation rules from an external storage, which can be helpful for evaluation rule management. This flagd PR may be helpful as well to see what metrics flagd supports

[thomaspoignant]

@Rongtao-Fu-Imprivata I have opened this pull request #555 that is providing a /metrics endpoint that is exposing the prometheus metrics.

Can you have a look at the pull request and tell me if it is aligned with what you had in mind?