Feature request: Support StructuredArgument

[knittl]

Support something similar to what logstash-logback-encoder offers, namely "Structured arguments", which allow easy adding of fields to your documents sent to Elasticsearch.

This is already possible when putting fields into the MDC with the correct encoder, but populating and then clearing the MDC is cumbersome.

Sample usage:

LOG.debug("Rendering took {} ms for {}", StructuredArguments.value("duration.ms", durationMs), StructuredArguments.keyValue("username", "userid"));

(Might be related to #31)

[ntvf]

Pull request for this one #65

[knittl]

I started porting over the code from https://github.com/logstash/logstash-logback-encoder in https://github.com/knittl/logback-elasticsearch-appender/tree/feature/structured-arguments but I am still unsure whether or not this is OK from a license compatibility standpoint.

@philsttr can you say something regarding this topic?

[philsttr]

I'm definitely not a lawyer, but I assume you just need to follow the conditions in Section 4. Redistribution of Apache License 2.0 (which is the license used by logstash-logback-encoder)

Alternatively, you could make logback-elasticsearch-appender depend on logstash-logback-encoder to take advantage of all of its JsonProviders and Structured Arguments without having to copy/paste.