使用例

using UnityEditor;
using UnityEditor.Experimental;
using UnityEngine;

public static class Example
{
    [MenuItem( "Tools/Hoge" )]
    private static void Hoge()
    {
        var imported = AssetDatabaseExperimental.counters.import.imported;
        var total    = imported.total;

        Debug.Log( total );
    }
}