Yes, this input plugin is for performance monitoring of the mongodb engine itself - not querying the database.
AFAIK there is no Telegraf input plugin available yet, that can query a mongodb database.
One possible solution would be:
With the inputs.execd Plugin you can include your own “plugin” in any programming language.
If you are fluent in Python, this should not be a big problem and be done in a few lines of code.
