private static DocumentOperationMessageV3 newPutMessage(FeedOperation op, String operationId) {
        var msg = new PutDocumentMessage(op.getDocumentPut());

        String id = (operationId == null) ? msg.getDocumentPut().getId().toString() : operationId;
        return new DocumentOperationMessageV3(id, msg);
    }
Keeps an operation with its message.

@author Haakon Dybdahl
vespaclient-container-plugin/src/main/java/com/yahoo/vespa/http/server/DocumentOperationMessageV3.java