Add fields for source

[felixbarny]

Map the source of the log event (class, function, line number) to ECS fields. Currently, there are no appropriate fields in ECS.

See also the discussion in elastic/ecs#281 (comment)

[felixbarny]

Adding these fields should be optional and off by default. It adds both runtime and disk overhead to gather and store that information for each log event.

[felixbarny]

I have created elastic/ecs#563 for that