For fields in EUCTR (protocol- and results-related information),
see also the register's documentation at
https://eudract.ema.europa.eu/.
For fields in CTGOV (protocol-related information), see also
the register's definitions at
https://prsinfo.clinicaltrials.gov/definitions.html.
Note: Generating a list of fields with this function may take
some time, and may involve running a mapreduce function if using
a MongoDB server. If the user is not not authorized to run
such a function, random documents are sampled to generate a
list of fields.