ore/metrics: implement Borrow for drop wrapper types

[teskje]

This PR adds Borrow implementations to the wrapper types DeleteOnDropHistogram, DeleteOnDropCounter, and DeleteOnDropGauge.

This makes it possible to write code that is generic over both the prometheus type and the wrapped type. The existing Deref impl does not work for this.

Slack thread for context.

Blocks #19964.

Motivation

• This PR adds a feature that has not yet been specified.

Make it possible to write code that is generic over both GenericGauge and DeleteOnDropGauge.

Tips for reviewer

@petrosagg points out that perhaps we shouldn't do this because Borrow is reserved for smart pointers. I would have called the DeleteOnDrop* wrappers smart pointers, but I'm also not clear about the definition. In any case, those types already implement Deref, which is also a smart-pointer trait.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • N/A

[petrosagg]

I see in the documentation of the Borrow trait a suggestion of using AsRef if the additional guarnatees of Borrow are not needed. Could we maybe add an AsRef impl to the types that we need?

If not, I think we should also add a comment in the definition of the struct warning future developers that they should not add a derive for any of Eq, Ord and Hash because that would invalidate the expectations of the Borrow impl

[teskje]

Could we maybe add an AsRef impl to the types that we need?

I don't think that would work. The goal would be to specify a trait bound like:

G: AsRef<GenericGauge<_>>,

... and then be able to pass both DeleteOnDropGauge and GenericGauge in place of G. But there is no blanket T: AsRef<T> impl, so you'd end up in the same situation as with Deref.

Adding a comment is a good idea!

[teskje]

Alternatively, we could just invent our own AsMetric trait if people would feel more at ease with that. Implementing Borrow is a bit scary.

But if we decide that the DeleteOnDrop* types are not smart pointers, we should probably also stop implementing Deref for them.

[teskje]

I've added the comments as suggested by @petrosagg. In Slack we seem to have arrived at a rough consensus that the DeleteOnDrop wrappers are something smart-pointer like ("smart handles"?), but also that the Borrow docs do not mention smart pointers anyway. So I think adding Borrow impls is fine here.

[teskje]

TFTR!