fields with dots in their name can overwrite each other

[joegallo]

With this config, I see my.field3 for each of my log events, but my.field1 and my.field2 don't appear. If I remove the fields definition for the prospector, however, then my.field1 and my.field2 do appear.

filebeat.prospectors:
- input_type: log
  paths:
    - foobar.log
  fields_under_root: true
  fields:
    my.field3: "field3"

fields_under_root: true
fields:
  my.field1: "field1"
  my.field2: "field2"

Likewise, if I use underscores instead of dots as separators (e.g. my_field1), then everything works as I'd expect.

• Version: 5.2.1
• Operating System: OSX, Linux
• Steps to Reproduce:

[tsg]

Marking it as a bug, even though the code actually works as initially intended, because we should revisit if the current behavior is the one we want.

[joegallo]

Another workaround for this is to repeat the global fields in each location where they would otherwise be lost due to the overwriting, e.g.:

filebeat.prospectors:
- input_type: log
  paths:
    - foobar.log
  fields_under_root: true
  fields:
    my.field1: "field1" # repeated
    my.field2: "field2" # repeated
    my.field3: "field3"

- input_type: log
  paths:
    - bazquux.log
  # don't need to repeat field1 and field2 here, it'll just work since there's no field3 to overwrite them

fields_under_root: true
fields:
  my.field1: "field1"
  my.field2: "field2"