Registry::register should not automatically append a period to the help text

[08d2]

Help strings should end with a period, per Prometheus guidance. But if a user does that themselves, the help string will be incorrect, as "a full stop punctuation mark (.) is automatically added to the passed help text" given to Registry::register.

Probably the best solution here is to append the period only if it doesn't already exist.

[mxinden]

Added to #52. Thanks @08d2. Let me know in case you want to tackle any of these.

[08d2]

I'm suggesting that help strings should be allowed to end in periods without that resulting in .. on scrape. Happy to PR if that's what it takes 👍