The original purpose of this change was to gate metrics that are unsupported by
some systems behind conditional compilation directives (#[cfg]); see
#6675 (comment).

Then I started doing some random cleanups and simplifications and got a bit
carried away. 🙈 The code should be overall tidier than before.

Changes:
- Don't register unsupported metrics (e.g. `max_rss` on non-Linux systems)
- Introduce `PrepareStats` struct as an abstraction over the `Ok` values of
  `PrepareResult`. It is cleaner, and can be easily modified in the future.
- Other small changes