Capture Logback KeyValuePairs

[jack-berg]

If I use the SLF4J 2.0.0 fluent API to do structured logging as follows:

            slf4jLogger
                .atInfo()
                .setMessage("A slf4j structured message")
                .addKeyValue("key", "value")
                .log()

I expect the key value pairs I add via addKeyValue(...) to be included in OpenTelemetry LogRecords when I use the logback appender.

Key value pairs are currently dropped. This appears to happen because the logback appender supports logback v1+, and a new API ILoggingEvent#getKeyValuePairs was added in logback v1.3.

We should add support to the logback appender to capture keyvalue pairs.

Note, this isn't a problem if using log4j2 as an implementation of slf4j2 via log4j-slf4j2-impl. Slf4j2 key value pairs are captured in log4j2 context data, which is accessed via LogEvent#getContextData(). There's already a property in the log4j2 appender config to capture context data.