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