#1132 Remove need for metadata tag for logs

[d-perl]

To test:

1. See that test_messages_are_tagged_with_run_uid still passes even without the plan metadata

[DominicOram]

Thanks, this looks cleaner. But whilst we're here:

• What is create_and_set_new doing on the RunUIDFilter? I don't think it's used.
• Could: I wonder whether we can combine the DCGIDFilter and RunUIDFilter classes into some kind of AppendMetaDataFilter which takes the name of the parameter to append as on __init__ but then still have separate instances for run_uid and dc_group?

[DominicOram]

Great, thanks!