Create endpoint for heap usage metric

[danielfireman]

Create a metric to return the heap usage. This metric should return a number between 0 and 100 which could be used in auto-scalability rules. For instance, the proxy could either publish or be queried by a cronjob which publishes custom metrics to AWS.

The returned number should be bigger as it gets closer to the GC to happen and smaller otherwise. So, it should be near zero just after the GC and grow monotonically as new objects are being allocated to the heap. It should be 100 when it is time to run the GC.