add cgroup memory.max_usage_in_bytes into slave monitor/statistics endpoint

As an operator, I'm periodically checking slave's monitor/statistics endpoint to get the memory/cpu usage/cpu throttle for each running task. However, if there is a short-term memory usage peak (lets say seconds), I might miss it (the memory might have been allocated and also released in between my 2 collect-metrics intervals). Since the max used memory is logged in the `/sys/fs/cgroup/memory/docker/CID/memory.max_usage_in_bytes`, it would be great, if this info would have been exposed in the api as well..