Khronus metrics reporter

[crypticmind]

This adds a new reporter to send histogram, counter, gauge, trace metrics to Khronus.

Currently, this uses com.despegar:khronus-java-client which has a dependency on Apache's HttpClient. Perhaps in a future release the underlying Khronus client can be swapped.

The reference.conf file has sensible defaults pointing to a Khronus instance running on localhost. To make this baby work in a service, this is the minimum configuration required:

kamon {
  khronus {
    host = "1.2.3.4:5678"
    app-name = "your-service-name-here"
  }
  modules {
    kamon-khronus.auto-start = yes
  }
}

[crypticmind]

@ivantopo please come 'ere and tell me your thoughts

[crypticmind]

I've just updated the implementation to also subscribe to executor-service metrics

[dpsoft]

@crypticmind the PR looks good for me, meanwhile just need to ask you please change the message to something like + add Khronus metrics reporter and will merge right away, just to match our commit message conventions. I would like to merge it BUT there is one missing piece, not in this repository but in our documentation website! yes, I know, it is a bit boring to write docs but it is a necessary thing... so if you could create a simple one page doc explaining what this module does, how it reports data and possibly some screenshots of what kind of visualization or results you can get out of it would be super helpful even for us, to start experimenting with this :D.. take a look at the Datadog, StatsD and New Relic pages on our website to get an idea of what we would like to see there.

BTW: @ivantopo WDYT

[crypticmind]

I've updated this PR and also created another one for the site.

[dpsoft]

@crypticmind thank for this PR!!!!!