src: simplify http2 perf tracking code

Use `unique_ptr`s and use the resulting simplification to
reduce indentation in these functions.

PR-URL: https://github.com/nodejs/node/pull/19470


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>