http2: metadata behavior when disallowed

[paul-r-gall]

Title: How to handle HTTP2 metadata when allow_metadata is false.

Description:
See #37152 (comment). When allow_metadata is false, new default behavior when Envoy attempts to send Metadata is to ENVOY_BUG and drop the metadata on the ground. It's debatable as to whether that's the correct behavior or not. We could potentially expand allow_metadata from a Bool to a more general purpose enum that looks like (pardon my syntax)

enum H2MetadataBehavior {
  DROP
  DROP_AND_ENVOY_BUG
  ALLOW
  ASSERT_FAIL
}

etc.

[RyanTheOptimist]

@birenroy @alyssawilk