using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        List<Data> dataList = new List<Data>

        {

            new Data("A",   "T1",   "MAU",  100),

            new Data("A",   "T1",   "WAU",  70),

            new Data("B",   "T2",   "MAU",  50),

            new Data("B",   "T2",   "Retention",    30),

        };

        var result = dataList

            .GroupBy(x => new { x.Org, x.Tenant },

                    (key, g) => new 

                     {

                        Org = key.Org,

                        Tenant = key.Tenant,

                        CustomData = g.ToDictionary(y => y.Dimension, y => y.Count)

                    })

            .ToList();

        Console.WriteLine(JsonConvert.SerializeObject(result, new JsonSerializerSettings {

            Formatting = Formatting.Indented    

        }));

    }

}

​

public record Data(string Org, string Tenant, string Dimension, int Count);