Clean-up fetch metrics tracking (#64746)
This ensures we only track fetch metrics in development mode as that's
the only time we report them currently, this also adds an upper limit on
how many metrics we store per-request as previously this was unbounded,
on top of that this ensures we don't keep tracking fetch metrics after
the request has ended as we only report on request end, and finally this
adds a clean-up to the reference we attach to the request object
containing the fetch metrics once we have used them.
Closes: https://github.com/vercel/next.js/issues/64212
Closes NEXT-3159
