Properly validate label names

[ttk]

The label name validation function was using the same regex as for the metric names. But according to the official prometheus docs the label names need to conform to a slightly different pattern. A check is also done for reserved label names; any that start with a double underscore (__).

Unit tests added and pass.

[ttk]

Not sure why phpcs code checks are erroring out on files that I haven't edited....Have the code check rules changed recently?

[LKaemmerling]

Hey @ttk,

thanks for your contribution! Looks good from my side!