add TCP connections status and UDP socket count plugin

[shirou]

Add tcp connection status plugin.
This plugin counts TCP Connection status such as established, time_wait, listen and so on.

related to #235

This plugin only counts the number. gopsutil can get family, local/remote address, port and so on. But I can not imagine how to use these information from telegraf. If you have any idea, I can add.

[sparrc]

@shirou thank you for the contribution, but it looks like you've broken some unit tests by adding a function to the PS interface. (see MockPS).

Also, can you add some simple unit tests for this plugin?

[sparrc]

Also, can you please write a README showing the metrics this collects? See #177

[sparrc]

you can use the cpu plugin as an example: https://github.com/influxdb/telegraf/blob/master/plugins/system/CPU_README.md

[shirou]

Roger. I will write a test and a README. please wait until this weekend.

[shirou]

@sparrc I added mockPS and README. Could you check these out?

However, I know my English is terrible, I would appreciate if you fix my English, and add more good explanation about TCP connection state.

[sparrc]

Thanks @shirou, but looking at your plugin, doesn't this also collect UDP information as well? Maybe you should generalize the name to something like netstat

I know that the current state of the system unit tests is a bit confusing (sorry!), but you should be able to accomplish it by modifying the data structure like this: https://github.com/influxdb/telegraf/blob/master/plugins/system/system_test.go#L54-L61

And then check down below like this: https://github.com/influxdb/telegraf/blob/master/plugins/system/system_test.go#L183-L193

[shirou]

@sparrc Sorry for anonying you. I couldn't get you. Now I added some test to system_test.go.
And as you said, I have added UDP socket counts to measure, and renamed to netstat.