ext_authz: Support filter naming and per-filter statistics

[danielmittelman]

Title: Support filter naming and per-filter statistics

Description:
My HTTP filter chain contains multiple instances of ext_authz, each fulfilling a completely different task.

My real-life use case:

http_filters:
  # Filter #1 - WAF sidecar
  - name: envoy.filters.http.ext_authz
    config: {...}

  # Filter #2 - request authorization service
  - name: envoy.filters.http.ext_authz
    config: {...}

In this case, I can access relevant statistics about the External Authorization executions in general, but I cannot distinguish between the two external services, and most importantly I cannot measure the allow/deny rates of each of the filters.

My suggestion is to have the option of naming each of the filters, and having that name added as a dimension to the statistics produced by the filter, for example:

http_filters:
  # Filter #1 - WAF sidecar
  - name: envoy.filters.http.ext_authz
    config:
      filter_stat_name: waf
      ...

  # Filter #2 - request authorization service
  - name: envoy.filters.http.ext_authz
    config:
      filter_stat_name: authorizer
      ...

[dio]

I think this is a reasonable ask. And i believe saw some TODOs related to this.