[NGRINDER-569] 2000 chracter for agent stat

Added exclude strategy during converting agent status map to json string.
Now only these 5 fields:
freeMemory, totalMemory, cpuUsedPercentage, recievedPerSec, sentPerSec
will be converted into string and saved in DB.
[NGRINDER-569] 2000 chracter for agent stat

Added exclude strategy during converting agent status map to json
string.
Now only these 5 fields:
freeMemory, totalMemory, cpuUsedPercentage, recievedPerSec, sentPerSec
will be converted into string and saved in DB.