db.coll.insert({id: 'foo', a: 100, b: 200, c: 300, d: {e: 400, f:500}})
db.coll.aggregate([
  {$docUnwind: {id: 0}},  // syntax similar to $project for whitelisting or blacklisting which fields to unwind
])
{id: 'foo', f_name: 'a', f_value: 100}
{id: 'foo', f_name: 'b', f_value: 200}
{id: 'foo', f_name: 'c', f_value: 300}
{id: 'foo', f_name: 'd.e', f_value: 400}
{id: 'foo', f_name: 'd.f', f_value: 500}