In several cases, we wish to add or remove metadata when we already know what
kind of metadata it is (e.g. we know we're dealing with the path, or the
authority, or the user agent, etc.).

In these cases, we do not need to re-compute the metadata batch callout index
since we know it a-priori. This saves us some branches and ALU ops spent
pointlessly re-computing these indices in several hot-path filters.

We do need the original methods where we do compute the indices in cases where
we're operating over a collection of metadata, but this is relatively uncommon.