tl;dr #5656 inspired a few of us I to take a look at the HCM to figure out if we could add generic useful features to the HCM to work around the issues described in the review of that PR, or if we should just do one-off complexity in the metadata handling.

We're leaning towards extending the filter return codes to allow for more flexibility, as well as debating how to handle filter "callbacks" which most filter implementer don't care about (e.g. metadata, 100 continue). Options that we've come up with thusfar are documented here:

https://docs.google.com/document/d/16istepO8qfG09UYdgxVzCOwJcjVL9Ua_AtcwJPyJUGI/edit?ts=5c59a7cf

(comments should be open to the public unless I screwed up permissions). If you're interested in the internals of how the HTTPConnectionManager manages streams and communicates to filters, please weigh, in as we'd like to sort out a plan and start hacking away :-)