Process Json log events

[bric3]

Currently the code understand a line as a log event message. However in some production systems, the application can use structured logging using a Json document. The document may contains many additional metadata, or context data, but what drain is interested in is the message, for this reason it has to be able to extract the message from the document given a path.

Usually log events are serialized in a single line json document, see logstash-logback-encoder for example. So when parsing json, the events will be assumed to be single line. However the message itself may be multiline (new line are likely encoded as \n).

So the only work to do is to pre-process the string line as a Json and extract the message field.