Enhancement: Do not overwrite requestId if extra already has one

This PR updates the `Processor\RequestId` so it does not overwrite a value for `requestId` in the `extra` array.

The reason I'm proposing this change is because I want to be able to pass on the `requestId` identifier, for example, to a `Gearman` worker, so I can identify log entries related to the request that started a background job.