Ping details of "unknown" ping don't include document type #226
Description
PR #194 added a metdata field document_type to show the ping type (original issue #79).
Unfortunately for new pings that are not known to the pipeline (and thus are coming in as invalid ones) the metadata is not added, and thus we don't get a document_type field.
The overview page has the ping type, if that information is also available in the detail view we should surface it.