add the kvictoria plug-in to provide metrics

[vrischmann]

This adds a plug-in to provide metrics using the VictoriaMetrics/metrics library as discussed in #875

It's based on the kprom plug-in although I decided to not provide any option other than the possibility to set the subsystem: in my experience, every metrics available in the hooks are useful and I see no point in not including them (and if a user doesn't want some metrics they can drop them during scraping).

There's no change needed in kgo but like I mentioned in #875, the PR #820 will be useful for this plug-in too because I'll be able to use the client id in more metrics; right now I can only use the client id in the various gauges tracking what's buffered.

I tested this locally with a small test program, a VictoriaMetrics instance that scrape it and a grafana dashboard to see if the metrics make sense. I can provide that if needed; as far as I can tell everything looks good.

Let me know if you have any question.

[twmb]

Is this forward compatible if somebody does want to add options to filter within the package in the future?
Should this wait for #820?

[vrischmann]

Is this forward compatible if somebody does want to add options to filter within the package in the future? Should this wait for #820?

Yes, if in the future someone adds these options it won't have any impact on current users since they'll have to be explicitly added in the NewMetrics call by the user.

It can wait for #820, in that case you wouldn't have to do two reviews (this one and a future one that uses the newly added ClientID). Personally I'm fine either way.