Fix indexcounters and globallock in 2.2 and 2.4 version

btree sub-document has been removed in 2.4 :
http://docs.mongodb.org/manual/reference/command/serverStatus/#serverStatus.indexCounters

ratio sub-document has been removed in 2.2
http://docs.mongodb.org/manual/reference/command/serverStatus/#serverStatus.globalLock.ratio