[{"_id":"5ded1a559015155eb8295f48","password":"20EABE5D64B0E216796E834F52D61FD0B70332FC:2512537"}]

db.collection.aggregate([{$addFields:{colonPos:{$indexOfBytes:["$password",":"]}}},{$addFields:{password:{$substr:["$password",0,"$colonPos"]},popularity:{$substr:["$password","$colonPos",{$strLenBytes:"$password"}]}}}])