adding open sockets

[rgarcia]

We're trying to extend this plugin to report on the number of open sockets that a container has open, but running into problems getting it to work.

I've verified that the metric is calculated correctly by setting Verbose to true and seeing it show up along side the other metrics computed by the plugin, e.g. logs like:

[2017-05-05 04:11:45] docker : Value parameters to be emitted:
container : 8b364b4/ecs-agent
dimensions : {}
point_type : custom.open_sockets
value : [0]
type_instance : None 
time : <module 'time' (built-in)>

However, I don't see this metric in SignalFX, and if I turn on the CSV plugin to write additionally write all metrics to file, I don't see the custom metric appear there.

I've submitted this PR mostly for guidance, but if you think it's generally useful I'm happy to clean it up for a merge.