Title: clarification around jw_authn metrics through docs

Description:

Started using envoy's jwt_authn http filter and was tracking down some additional stuff for metrics.

Found them in the repo. It will be nice if we could have some docs exposing these metrics. A lot of them seem pretty self-explanatory but some are a bit ambiguous

http.ingress_http.jwt_authn.allowed
http.ingress_http.jwt_authn.cors_preflight_bypassed
http.ingress_http.jwt_authn.denied
http.ingress_http.jwt_authn.jwks_fetch_failed
http.ingress_http.jwt_authn.jwks_fetch_success
http.ingress_http.jwt_authn.jwt_cache_hit
http.ingress_http.jwt_authn.jwt_cache_miss

[optional Relevant Links:]

• Some example of cache hit metrics