Receipt of GRIB keys written via encoder

[HCookie]

Is your feature request related to a problem? Please describe.

When writing data to a sink with multio and combining the encoder and scale actions it becomes a little unclear the exact keys that were written.

plans:
        - name: output-to-file
          actions:
          - type: scale
            preset-mappings: local-to-wmo
          - type: encode-mtg2
            geo-from-atlas: true
          - type: sink
           	...

Particularly with the mappings, the param id can change.

Describe the solution you'd like

In some applications having a receipt of what was written would enable easy mars / fdb archival.
Some sort of mars translatable spec would be helpful here.

This is needed for a simplification of the operational suite of the AIFS, but not for the immediately next version

Organisation

ECMWF