Support for custom metadata for logs sent through LogForwarding

Proposed title of this feature request

  Additional Custom Log Metadata

What is the nature and description of the request?

  Customer would like a way to add metadata fields in the log messages gathered by fluentd. This would be a constant value set on each log. Historically they have used something like:

 <filter project.**>
  @type record_transformer
   <record>
    cluster_name my_cluster_identifier
   </record>
 </filter>

Why does the customer need this? (List the business requirements here)

  Customer sends logs through LogForwarding from multiple clusters. They'd like to add a custom metadata field which is hard set to the cluster name so they can more easily search in their external log store.

They would like a way to do this in a supported way with a Managed ClusterLogging instance