feat: Client improvements

[slowli]

What ❔

Improves the client crate in various ways:

• Supports Gauges for more types, e.g. usize / isize.
• Supports raw identifiers (e.g., r#type) as label names.
• Supports encoding C-style enums as label values and transforming their variant names like in serde.

Why ❔

Makes the crate easier to use (with the guiding example being the main Era repo).

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted and linted using cargo fmt and cargo clippy.

[popzxc]

Do I get it right that this assumes that the ident is always in UpperCamelCase?
If so, is it also expected that acronyms are always treated like words?
E.g. looked like this would transform HTTPStatus to h_t_t_p_status via SnakeCase.

[popzxc]

Also, do we want to break on digits? E.g. Status500 -> status_500?

[slowli]

I've consulted the serde impl 🙃 It does have the peculiarities you've described:

• It assumes that the input is PascalCase. IMO, it's a reasonable assumption to make; I guess we can document it.
• Acronyms are tricky. IIRC, Rust best practices recommend to capitalize only the first letter of acronyms (i.e., HttpStatus rather than HTTPStatus). In any case, in these situations one can specify the name override explicitly.
• Breaking on digits is tricky as well; I feel there's no obvious way to insert or not insert a _ before them. Again, it's possible to specify the name override to make it explicit.

[popzxc]

Yeah, I don't suggest changing anything, just wanted to check whether this was considered.
Probably it indeed makes sense to document this behavior.

IIRC, Rust best practices recommend to capitalize only the first letter of acronyms

Well, yeah, there is a clippy lint for that, but it's ranked 4th in the list of most commonly ignored lints. Looks like the community doesn't agree with this best practice :)