Add support for Metrics / Stats API

[alvarolobato]

Work is underway to add support for Metrics to the API/server: elastic/apm-server#964
Support needs to be added to this agent too.

The default minimum set of metrics should be supported. TBD which additional metrics will be supported for this agent.

[cah-andrew-fitzgerald]

Are there plans for this to work with existing metrics libraries?
Would be nice for this to support micrometer, which is the default for Spring.

[felixbarny]

Micrometer is definitely on our radar and we'll evaluate possible integrations.

[jkschneider]

I'd love to see a spec around what the apm-server expects, as for Micrometer ideally we'd support shipping directly to API rather than through the agent. Agents aren't altogether bad, but they can be unacceptable in certain environments.

[felixbarny]

There is no such spec yet, but you will be kept up-to-date by subscribing to this issue.

[jkschneider]

@eyalkoren That doc is not shared.

[eyalkoren]

@jkschneider right, thanks! It's not shared because it is still WIP, my fault of referring to it from here. In any case, we may start with just supporting basic JMX stuff and maybe extend later on to do some integrations

[felixbarny]

This PR added basic support for metrics: #360

Note that there is no public API to add metrics. Instead of building an API ourselves, I think it makes more sense to write an Elastic APM reporter for micrometer.