Hi Praveen,
Upgrade dbagent from 4.1.8.7 to 4.2.x latest db agent and see how it goes and also check whethre the user running db agent have acccess to mongodb admin for below url and check permissions link below and see if that information helps
localhost:27017/admin
https://docs.appdynamics.com/display/PRO42/MongoDB+Database+Permissions
02:36:02,110 INFO [main] DBAgent:46 - Using Agent Version [Database Agent v4.1.8.7 GA Build Date 2016-01-12]