sum two fields in documents belonging to a specific value

Question

My document is:

  [
   { '_id': 1, number: 10, span: 5, rated:'Yes', sectionID: 3},
   { '_id': 2, number: 20, span: 6, rated:'Yes', sectionID: 3},
   { '_id': 3, number: 4, span: 5, rated:'No', sectionID: 5},
   { '_id': 4, number: 2, span: 0, rated:'Yes', sectionID: 6},
   { '_id': 22, number: 9, span: 6, rated:'No', sectionID: 3},
  ]

I want to sum number and span fields for sectionID 3 when they are rated

So the result will be like:

     number: 30, span: 11

I have looked at this great post but couldn't wrap my head around working with two fields at once.