nstat input plugin

[maksadbek]

Plugin discussed here: #1104

@sparrc Plugin for nstat, could not use your nstat library because there was a bit misfit and required refactoring, but I used the your parsers.

[sparrc]

Here I think we should do the same thing nstat does: https://github.com/shemminger/iproute2/blob/master/misc/nstat.c#L46,

which is:

1. Use overrides here (but default to "")
2. try PROC_NET_NETSTAT env var
3. try PROC_ROOT env var + "net/netstat"
4. use "/proc/net/netstat"

[maksadbek]

Hi @sparrc, I have fixed code, the paths for files are read as follows:

1. Try to read from config, default is ""
2. Try to read paths from env
3. Try to read PROC_ROOT and append default paths
4. use default paths: /proc/net/netstat, /proc/net/snmp, /proc/net/snmp6

Wrote detailed documentation, what fields are collected and their tags.
Feel free to comment if I missed smth, any your suggestions are appreciated.