Implementing a Recordable interface for Logs

[kxyr]

Problem
An important use case of logging is to allow log records to be sent to multiple export destinations, as seen in these logging libraries:

• Log4cxx
• Plog
• Log4J

In the current Logging SDK, a LogRecord is created as a unique_pointer by the SDK, then moved to a processor, then passed by const reference to an exporter. This implementation does not allow more than one processor per Logger.

Solution
One solution could be to implement a Recordable interface, similar to what is currently used for traces. The recordable idea outlined here.

Alternatives Considered
Another possible (simpler but less performant) solution is to pass a LogRecord as a shared_ptr to be shared between the multiple processors/exporters. However, this approach could offer inconsistent performance related to memory-management which is a goal.

Additional Context
Further discussion about the Recordable interface can be found in #44 and #49.

cc @alolita @MarkSeufert

[meastp]

@xukaren I don't know if you think it's relevant, as you already have a multiple examples, but Boost.Log is also designed around multiple sinks / export destinations as seen in the Boost.Log design overview documentation.

[kxyr]

Resolved by #438