As frequent-but-not-every-day-one-needs-these-command MongoDB users, we, obviously, can use a cheat sheet. Here is the start of one.
Since not all documents may have all keys, the following method loops over all documents and collects the encountered keys.
var dbks = {};
db.___your-collection-name___.find().forEach(function(doc){Object.keys(doc).forEach(function(key){allKeys[key]=1})})
dbks;
2021-02-03: more ...