access_log: formatter for upstream host metadata (#21724)

Part of #20389. Most of the formatters used in header manipulation are also present in substitution access log formatters. However, UPSTREAM_METADATA was not present in access log formatters.

Also, as noted in #17457 all xxxx_METADATA will be eventually replaced my METADATA(xxxx,...) so this PR also extends METADATA formatter.

api changes are trivial and limited to comments.
Risk Level: Low
Testing: Added unit tests.
Docs Changes: Yes.
Release Notes: Yes

Signed-off-by: Christoph Pakulski <christoph@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ 380a328f7e32e4d18d31cba82bac7143904e0536

envoy/extensions/formatter/metadata/v3/metadata.proto

@@ -20,6 +20,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // * DYNAMIC
 // * CLUSTER
 // * ROUTE
+// * UPSTREAM_HOST
 //
 // See :ref:`here <config_access_log>` for more information on access log configuration.
 
@@ -51,7 +52,8 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 //  .. note::
 //
 //    METADATA(DYNAMIC:NAMESPACE:KEY):Z is equivalent to :ref:`DYNAMIC_METADATA(NAMESPACE:KEY):Z<config_access_log_format_dynamic_metadata>`
-//    METADATA(CLUSTER:NAMESPACE:KEY):Z is equivalent to :ref:`CLUSTER_METADATA(NAMASPACE:KEY):Z<config_access_log_format_cluster_metadata>`
+//    METADATA(CLUSTER:NAMESPACE:KEY):Z is equivalent to :ref:`CLUSTER_METADATA(NAMESPACE:KEY):Z<config_access_log_format_cluster_metadata>`
+//    METADATA(UPSTREAM_HOST:NAMESPACE:KEY):Z is equivalent to :ref:`UPSTREAM_METADATA(NAMESPACE:KEY):Z<config_access_log_format_upstream_host_metadata>`
 
 message Metadata {
 }