Question Collapsible banner using AdMob

Hi there!
I want to try implement collapsible banner for Unity game. I'm using Google Mobile Ads official SDK.
I didn't find any official Google guides about implementing this extension for banner ad even for native, only a few threads on some forums like here and here.

I tried to pass collapsible arguments to request Extras:

Code (CSharp):

1. new AdRequest.Builder()
2.     .AddExtra("max_ad_content_rating", adSettings.ContentRating) // works fine
3.     .AddExtra("collapsible", "bottom") // do nothing
4.     .Build();

But as I understand the native examples by links above I need pass this extra settings not to request but to AdMob network directly by using

AddMediationExtras

builder extension. I've created the next implementation with parameters I need:

Code (CSharp):

1.         public class CollapsibleBannerMediationExtras : MediationExtras
2.         {
3.             // I need to test it firstly on Android so null string here
4.             public override string IOSMediationExtraBuilderClassName { get; }
5.             public override string AndroidMediationExtraBuilderClassName => "com.google.ads.mediation.admob.AdMobAdapter";
6.             public CollapsibleBannerMediationExtras()
7.             {
8.                 Extras = new Dictionary<string, string>()
9.                 {
10.                     ["collapsible"] = "bottom"
11.                 };
12.             }
13.         }

And use it for building request:

Code (CSharp):

1. new AdRequest.Builder()
2.     .AddExtra("max_ad_content_rating", adSettings.ContentRating)
3.     .AddMediationExtras(new CollapsibleBannerMediationExtras())
4.     .Build();

But I get the runtime error message:

Code (CSharp):

1. UnityEngine.AndroidJavaException: java.lang.NoSuchMethodError: no non-static method with name='buildExtras' signature='(Ljava.util.HashMap;)Ljava/lang/Object;' in class Lcom.google.ads.mediation.admob.AdMobAdapter;

And, yeah, this class do not have the buildExtras method. So I think I should find the right class to pass it to extras implementation class, but I don't really know what class I need.

Or maybe this functionality cannot be achieved through Unity.

Could anyone please help me with that task ?
Thanks!