Support for sending health metrics over Loggregator

[boyang9527]

copied from app-autoscaler-release project. cloudfoundry/app-autoscaler-release#162

As a CF component it would be nice if auto scalar published health and metrics over loggregator. CF Operators have all had to integrate loggregator into their monitoring systems to monitor all of the CF Core Components. It would be nice if auto scalar supported the same metrics system as all the other Cf core components. This would allow operators who wished to add auto scalar to their system to not have to bother with monitor auto scalar different than any other CF component.

Today auto scalar provides a Prometheus endpoint which is great for those using Prometheus to monitor their Cloud Foundry Deployments but not so great for those who don't.

With the rewrite to golang that might simplify integration with loggregator since loggregator provides a common golang library for this purpose.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/168083471

The labels on this github issue will be updated when the story is started.