Undeterministic order in output

The output or encode() is non-deterministic when creating multiple metrics of the same type.

So basically:

pub fn repro() -> String {
    let mut registry = <prometheus_client::registry::Registry>::default();

    let gauge = Family::<MyLabels, Gauge<f64, AtomicU64>>::default();
    registry.register("test", "help", gauge.clone());

    gauge
        .get_or_create(&MyLabels {
            label: "one".into(),
        })
        .set(0.1);
    gauge
        .get_or_create(&MyLabels {
            label: "two".into(),
        })
        .set(0.2);

    let mut buffer = String::new();

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

    buffer
}

This will sometimes return this:

HELP test help.
# TYPE test gauge
test{label="one"} 0.1
test{label="two"} 0.2
# EOF

And sometimes this:

HELP test help.
# TYPE test gauge
test{label="two"} 0.2
test{label="one"} 0.1
# EOF

I would expect the metrics to be somehow sorted deterministically.

Start at encode and trace how the Registry and Family values are iterated when multiple metrics share a type. Use the provided repro to observe the ordering, then verify that repeated encodes produce the same metric order and match the expected deterministic output.