Add 'metadata' option to include metadata fields

[magnusbaeck]

Currently the only way (?) to include the metadata fields in an output is by using the rubydebug codec, which is fine if the consumer is human but if a program is to read the data that's not very useful. I suggest that the json_lines codec gets the same treatment. If this is accepted I'll make the same addition to the json codec.

A case in point is the Logstash Filter Verifier tool which currently can't be used to test Logstash configurations that output events with metadata fields set as is relies on being able to parse the Logstash output via the json_lines codec. Being able to test metadata fields was requested in magnusbaeck/logstash-filter-verifier#5.

[andrewvc]

I think this is a good idea @magnusbaeck !