[VOTE] PIP-304: Make PublishContext provide the message metadata

[kecona]

Hi all,

This thread is to start a vote for PIP-304.

PIP: https://github.com/apache/pulsar/pull/21311
Discussion thread:
https://lists.apache.org/thread/72szlzc0rw1hbn4v0w54qgq93cn9k4px

Thanks,
kecona