Provide threshold to automatically trigger dump creation

[tobespc]

As well as being able to manually request a dump, can we add some configurable thresholds that if reached (mem > xxxx for example) a dump is automatically triggered

[sam-github]

Seems like a useful feature of appmetrics itself, or some addon, but shouldn't require a UI dashboard. In particular, it shouldn't involve UI interaction with every single uservice after it comes up to configure the threshold. Would be nice to have this feature be remotely accessible via protocol, and to be configurable via env var.

This would be an easy feature to add into strong-supervisor, but it seems like it is more fundamental than that. If we aren't recommending strong-supervisor for all runtimes, then the feature should be in appmetrics, or at least easy to add to appmetrics.

/to @cvignola, because liberty has similar features to this, and its the kind of feature that should be available uniformly in docker-deployed node.js runtimes, as well as bluemix.

[tobespc]

The core feature to do the actual triggering and would be in appmetrics. The UI would be able to configure it live for those users who do not want to use the api. For other use cases, we would have it configurable via the api/config files