Prometheus metrics don't follow best practices

[aabouzaid]

Hello,

Currently, KES Prometheus metrics (last_state and sync_calls) don't follow Prometheus metric names best practices.

That's because:

A metric name...

• ...should have a (single-word) application prefix relevant to the domain the metric belongs to. [...]
• ...should have a suffix describing the unit, in plural form. [...]

Without a prefix, suffix, or unites it's hard to identify KES metrics and work them.

I'd be happy to make a PR for this change, but since it's a breaking change, I'd like to hear which strategy to follow to deprecate the old metrics.

[lwille]

I'd suggest the following strategy to introduce the new metric and phase out the old one:

• implement new prefixed metric in addition to the old one
• a new metric doesn't break anything, so it can be released any time
• mark the "old" metric deprecated in the code and schedule its deletion for the next major version release

[Flydiverny]

Sounds good, PR welcome 😄
Ideally we could solve #399 as well, but think we need to introduce some more magic than just renaming for that 😬