Yes - this is frustrating.
Agent installs, mongodb integrates -- and the docs tell you how to add a mongodb user/password.
But, there is NOTHING documented on how to configure the agent integration to actually use a username + password when it connects to the mongodb.
come on, Elastic ... answer this dude's question.
--edit --
Looks like, if you dig around: check your policy for the agent, then the mongodb integration line. You should have some enabled "collect" functions, listed in rows, each with a triangle dropdown on the far right.
Oh ... somewhere in there, you need to click Advanced Settings link to open up the absolutely long list of configuration options for that function of the integration.
Keep going ... check each one of those, scrolling down until you stumble across the username and password form fields.
Lord! please document this common configuration steps or move the most obvious ones (like user/password that you documented to actually do), up to the initial integration-addition steps. ...