http2: improve JS-side debug logging

DRY up the `debug()` calls, and in particular, avoid building template
strings before we know whether we need to.

Backport-PR-URL: https://github.com/nodejs/node/pull/29124
PR-URL: https://github.com/nodejs/node/pull/29122


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>