Update HTTP/2 METADATA documentation. (#17637)
Update HTTP/2 METADATA documentation. (#17637)

This commit updates the documentation for HTTP/2 METADATA in Envoy,
particularly the class names referenced in the metadata flow.

Risk Level: n/a (docs)
Testing: n/a
Docs Changes: yes
Release Notes: no

Signed-off-by: Dianna Hu <33dbe63c766a11bad0faebf4e40938658fdcd414@google.com>

envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy