The instrument helper we had here would allocate a new string object for
the key each time it was called and also added two stack frames. The
interpolated strings are also _slightly_ slower to hash since they
aren't fstrings.

Though this was shorter before, I think this is a little more preferable
as a code style since the subscribed keys are now greppable. After this,
there's still one other place this pattern is used (in ActiveSupport
cache).