Add Mapped Diagnostic Context data to LogEvents as attributes

[jasonjkeller]

Some log libraries provide APIs for adding context data to log messages using MDC. Customers will likely want this data captured and included on LogEvents. We should explore capturing this data for supported log libraries.

Mapped Diagnostic Context (MDC)
A typical distributed system is multithreaded, with different threads serving different requests from different clients. Each thread has its own specific context. Contextual metadata in your logs—such as user ID, client host, session information, query data, etc.—can be invaluable to getting more insight into how your application is performing and which users are experiencing problems.

Log4J, Log4J2, and Logback all support Mapped Diagnostic Context, or MDC, which lets you store key-value pairs that are then added to your log data. This is particularly useful if you are logging to JSON because these contextual attributes can then be used to filter and sort your logs.

[StanlieK]

@jasonjkeller Hey! As you may have seen, I've created pull-request for this feature #866.