On mongod, we search the command registry for the command name specified in the request object. This ensures that aliases like "ismaster" are properly covered in the exhaust isMaster metrics.
On mongos, however, we only look at the command name passed into the request. This means that we will incorrectly mark inExhaustIsMaster as false if we send "ismaster" instead of "isMaster" to mongos.
Mongos should look up the request's command name in the command registry so that any aliases to the isMaster command are correctly covered.