feat(metrics): support Gauge<u32, AtomicU32> type

[koushiro]

support Gauge<u32, AtomicU32> metric

[koushiro]

@mxinden PTAL

[mxinden]

Cool. Thanks. Looks good to me.

Can you bump the crate version in the root's Cargo.toml to v0.22.2 and add a CHANGELOG.md entry? Once this pull request merged, I can then cut a new release right away.

[mxinden]

The following test:

    #[test]
    fn encode_gauge() {
        let mut registry = Registry::default();
        let gauge = Gauge::<u32, AtomicU32>::default();
        registry.register("my_gauge", "My gauge", gauge);

        let mut encoded = String::new();

        encode(&mut encoded, &registry).unwrap();

        parse_with_python_client(encoded);
    }

Would fail with:

   --> src/encoding/text.rs:635:51
    |
635 |         registry.register("my_gauge", "My gauge", gauge);
    |                  --------                         ^^^^^ the trait `EncodeGaugeValue` is not implemented for `u32`
    |                  |
    |                  required by a bound introduced by this call

Mind adding an implementation of EncodeGaugeValue for u32? You can cast into an i64 and then use the existing implementation for i64.

Please also extend the encode_gauge test in text.rs to register both a default Gauge and a Gauge<u32, AtomicU32 to make sure we don't break this in the future.

Sorry for the additional work. Thank you for the help.

[koushiro]

Thanks for your review, I have changed encoding related stuff and added a simple test case to encode_gauge.
@mxinden PTAL again

[mxinden]

Well done!

[mxinden]

Published and tagged.