Observability on external auth module

[eguzki]

In order to be able to observe what is happening at runtime via tools such as prometheus and grafana being able to expose some metrics around the WASM module would be useful.

As initial suggest, Envoy external authorization filter statistics module (source), exports the following:

Name	Type	Description
ok	Counter	Total responses from the filter.
error	Counter	Total errors contacting the external service..
denied	Counter	Total responses from the authorizations service that were to deny the traffic.
disabled	Counter	Total requests that are allowed without calling external services due to the filter is disabled.
failure_mode_allowed	Counter	Total requests that were error(s) but were allowed through because of failure_mode_allow set to true.

Related #58 and #6