Add a way to access a subset of metrics

Currently, the only way to access libprocess metrics is via the `metrics/snapshot` endpoint, which returns the current values of all installed metrics.

If the caller is only interested in a specific metric, or a subset of the metrics, this is wasteful in two ways: First the process has to do extra work to collect these metrics, and second the caller has to do extra work to filter out the unneeded metrics.

Ideally libprocess could use the request path to implement filtering such that e.g. a request to

wget http://mesos.master:5050/metrics/allocator/mesos/

would return all metrics whose name begins with "allocator/mesos/", but I'm not sure that this is currently implementable.

Alternatively, a request parameter could be added to the same effect:

wget http://mesos.master:5050/metrics/snapshot?prefix=allocator/mesos/