User can't supply arguments as files (for example credentials) to pmm-admin.
Steps to reproduce:
file argument is ignored.
this feature is not fully official, but was supported by kingpin and suggested in:
none, but there would be a special flag:
We use the mechanism described in PMM-8357 to put all flags in a file and use that file in config/add command. This worked perfectly fine in 2.29.1, but seems not to work with 2.31.0. The flags file and its content gets ignored.