We should add support for transient metadata fields that get added by ingest to each document that gets processed, which can later be referred to by any processor. For instance the ingest timestamp would be a good one to be available all the time; it should be possible to add a field through the add processor and set it to that value. Same for information about the error when a processor fails, so that the on failure processor can grab it and write it to the desired field using the add processor.

Yet to be defined is the syntax to refer to these fields, we have to be careful and try to prevent confusion with elasticsearch metadata fields.

Relates to #14990 .