in ClientMessageDtos.proto (NewEvent/EventRecord).metadata_content_type is required
However metadata is optional
Looks like a bug.
the metadata_content_type is not used at this time (its there for when the feature gets added to not break protos) and there will be a value for "unknown" 0 which happens to be the default :)
still, better not pass one more odd int32 over the network, let's define that with default on layer above at client/server side