What did you do?

Started consuming /federate?match[]= data by other tools, as though it were a normal prometheus-exporter format.

What did you expect to see?

HELP and TYPE metadata passed through from the upstream exporters.

What did you see instead? Under which circumstances?

Always
# TYPE .... untyped

This makes the federation facility lossy in terms of information, especially with respect to summary/histogram types.

This was with prometheus 2.0.0