Filebeat: retain original type of fields added in configuration

[robin13]

If you add a field in the prospector section of filebeat configuration

      fields:
        widget_number: 10

This will be added to each document as:

"widget_number":"10"

but it would be better if numbers were not cast as string, and would remain:

"widget_number":10

[DirkAmelinckx]

+1

[ruflin]

I assigned the libbeat label here as this would be a general improvement to the way fields are handled.

[andrewkroh]

This is the behavior in v5. Changed in #1092

From the 5.0 changelog:

• Scalar values in used in the fields configuration setting are no longer automatically converted to strings. {pull}1092[1092]