Adding Firebase Ads to android project in visual studio

I setup new Monogame android project via VS2017 community edition, select android 7 as my target
then i add via xamarin components firebase ads which adds a bunch of references of google and etc.
then i try to build the project and i see the following warning:
MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(1673,3): warning : Name cannot begin with the ‘$’ character, hexadecimal value 0x24.

Can anyone help with that?
my VS 2017 & xamarin are both up-to-date using xamarin android 7.2