fix: avoid mutation bug in registry

[Raynos]

The defaultLabels feature is buggy when used in combination
with the .labels() sub counter feature.

We mutate the object in the serialization implementation which
means the next time you observe the value the labels object has
been incorrectly mutated.

The functionality of .labels() is supposed to be the caching
of the labels object itself instead of constantly re-using it.

I may actually be the only one using the caching feature of the
.labels() method which is why this bug was not caught earlier.

The .labels() method is not broken if you use it as

histogram.labels(...).observe(int)

But is broken if you do

class Foo {
    constructor() {
        this.myHistogram = histogram.labels(...)
    }

    method() {
        this.myHistogram.observe(int)
    }
}

[SimenB]

Could you add a test and update the changelog as well, please?

Code itself LGTM 🙂

[Raynos]

I've updated changelog and added a test that fails without my patch.

[SimenB]

Thanks!

[SimenB]

https://github.com/siimon/prom-client/releases/tag/v11.5.2

[Raynos]

Awesome.