feat: unit test

[opensourcevk]

unit test for utility

[beorn7]

Thanks for your contribution.

However, I'm not sure if we really need a "test for the test". The code we use here is well established and not expected to ever change again.

@bboreham since you touched this in your refactoring, maybe you have an opinion?

[bboreham]

This package is used in Prometheus non-test code; I think it’s fine to check it stand-alone.

I have not studied the values used to see if they cover most corner cases.

[bboreham]

Needs DCO sign-off before we could merge.

[beorn7]

https://floating-point-gui.de/errors/NearlyEqualsTest.java gives some inspiration for corner cases (with the notable exception that our function returns true if it compares two NaNs (which BTW should probably be mentioned in its doc comment.

@opensourcevk could you do the following:

• Add a few more corner cases, those not yet covered from the link above.
• Add a doc comment to the Equal function that it considers all NaNs to be equal to each other.
• Squash your commits into one, give it a more descriptive description, and sign the DCO.

Thank you very much.