Not sure of your data in MongoDB, I take it that the "clients" column is some sort of list/array? You may have to convert whatever it is into something Power BI understands, like a Table or List.
For example, here are is some R code for processing information resulting from a Machine Learning call:
print("Result:")
result = h$value()
print(fromJSON(result))
##Return results back
inter <- do.call("rbind", finalResult$Results$output1$value$Values)
MyFinalResults <- data.frame(inter)
names(MyFinalResults) <- finalResult$Results$output1$value$ColumnNames
rm(list=setdiff(ls(), "MyFinalResults "))