Some MongoDB fields are wrong

[exekias]

Some mongodb fields do not match their field definition, we will need to update it. See:

https://github.com/elastic/beats/blob/master/metricbeat/module/mongodb/dbstats/data.go#L12 and
https://github.com/elastic/beats/blob/5.3/metricbeat/module/mongodb/dbstats/_meta/fields.yml#L7

.bytes is appended to fields definition but not reported by the code. We should review all the fields in this module, as I see others affected too.

This was detected in this thread: https://discuss.elastic.co/t/cannot-index-event-avg-obj-size/81941/1

[exekias]

We should update #3842 with the result of the fix for this