There are several reasons to extract this function:
* It's easy to unit test without extensive mocking.
* It will show up when we profile code.
* It is something that you can mostly ignore for
most messages.
The main reason to extract this, though, is that we are about
to do some fairly complex splicing of data for the use case
of mentioning service bots on streams they are not subscribed to,
and we want to localize the complexity.