chore: Update uses of prometheus registration to be less likely to panic

[dehaansa]

PR Description

Inspired by #2177 - I don't see a reason that the possibility of a metric being already registered should cause Alloy to panic.

Which issue(s) this PR fixes

None

[wildum]

lgtm

[ptodev]

It's worth noting that the AlreadyRegisteredError feature is more of an anti-pattern, and should only be used as a last resort. It's not meant to be used regularly for every registration. If there are duplicate registrations, there is likely to be a bug and avoiding the error could just further mask the bug.

To avoid the panic, it's possible to use Register instead of MustRegister and then the error can be propagated.