[ Feature Request ] send gather snmp time as Self Stats monitoring

[toni-moreno]

We are computing and logging for each device the time that the engine has took to get all data, could be interesting also send to the influx monitoring DB.

INFO[2016-10-28 05:54:58] snmpdevice [hostsnmpv3b] snmp pooling took [476.647899ms]  
INFO[2016-10-28 05:55:28] snmpdevice [hostsnmpv3b] snmp pooling took [488.502427ms]  
INFO[2016-10-28 05:55:58] snmpdevice [hostsnmpv3b] snmp pooling took [526.179549ms]  
INFO[2016-10-28 05:56:28] snmpdevice [hostsnmpv3b] snmp pooling took [579.377993ms]  
INFO[2016-10-28 05:56:58] snmpdevice [hostsnmpv3b] snmp pooling took [473.428373ms]  
INFO[2016-10-28 05:57:28] snmpdevice [hostsnmpv3b] snmp pooling took [527.389091ms]  
INFO[2016-10-28 05:57:58] snmpdevice [hostsnmpv3b] snmp pooling took [523.378573ms]  
INFO[2016-10-28 05:58:28] snmpdevice [hostsnmpv3b] snmp pooling took [510.147521ms]  
INFO[2016-10-28 05:58:58] snmpdevice [hostsnmpv3b] snmp pooling took [488.666555ms]  
INFO[2016-10-28 14:28:50] snmpdevice [hostsnmpv3b] snmp pooling took [334.849698ms]