[java-logging-logback] JsonPayload customization

We have a requirement to include custom fields in the Cloud Logging JsonPayload, in our case used by our datadog log sink integration for filtering and billing attribution of dataflow job logs.

In the current LoggingAppender implementation the JsonPayload is not modifiable and is already set on the LogEntry.Builder before invoking LoggingEnhancer.

When using Cloud Logging client directly we can build the custom JsonPayload.

Feature request to allow JsonPayload modification either via LoggingEnhancer or Logback encoder or some other mechanism.