Define built-in processor to add log.record.uid if missing

[jack-berg]

#3047 adds semantic conventions for log.record.uid. I proposed that it would be useful for the SDK to automatically add this attribute.

This is a natural use case for another built-in LogRecordProcessor.

Let's define a standard LogRecordProcessor which adds log.record.uid to LogRecords if it is not already present. It can add guids, or ulids, or perhaps be configurable with a custom id generator to allow users to generate their own ids.

[joaopgrassi]

I think this is a nice idea and would allow augmenting logs in the future if necessary. One thing I was thinking, would the addition to this cause any breaking change? Giving the Logs API/SDK is getting close to stable maybe it's worth considering?

[jack-berg]

I can't think of any reason why it can't be a backwards compatible addition.