Description

Envoy has metrics for filters to get an overview of the plugin execution statistics, we need to expose metrics for plugins.

Use Scenes

Just like the envoy filters, e.g. the global rate-limiting has metric statistics to describe how many requests has over the limitation. It can also help us for troubleshooting.

Advice

thanks to @spacewander for the suggestions below:
Envoy Go metric API: https://github.com/envoyproxy/envoy/blob/0ff8fc30ae72a5a0efdc7d651f217021c77056b3/contrib/golang/common/go/api/filter.go#L312
While htnn not inetgrated yet, we need to enhance this part: