The example at https://docs.opsmanager.mongodb.com/current/tutorial/configure-monitoring-agent-for-kerberos/#set-the-krb5principal :

krb5Principal=mmsagent/someserver.example.com@EXAMPLE.COM

may mislead the reader to think that the instance part of the principal must be a hostname. It may be better to replace "someserver.example.com" with "instance" or just have the whole thing be "mmsagent@EXAMPLE.COM"

See HELP-2003