// appsettings.json имеет вид 
{
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*",

  "General": {
    "EtcdMembersUri": "http://localhost:2380, http://localhost:2379",
    "EtcdLogin": "user",
    "EtcdPassword": "password"
  }

}

// Startup -------------------------------

    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            // 
            services.Configure<GeneralConfig>(Configuration.GetSection("AppSettings:General"));
                  ................................
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        }
   }

// класс для работы с конфигом

    public class GeneralConfig
    {
        public string EtcdMembersUri { get; set; }
        public string EtcdLogin { get; set; }
        public string EtcdPassword { get; set; }
    }

// Handler 


namespace MediatrWeb.Handlers
{
    public class Run_EtcdHandler : IRequestHandler<RequestEtcd, ResponseEtcd>
    {
        private readonly string _etcdMembersUri;
        private readonly string _etcdLogin;
        private readonly string _etcdPassword;

        public Run_EtcdHandler(IOptions<GeneralConfig> appSettings)
        {
            _etcdLogin = appSettings.Value.EtcdLogin;                        // <<<<< тут NULL  ?????????
            _etcdPassword = appSettings.Value.EtcdPassword;              // <<<<< тут NULL ?????????
            _etcdMembersUri = appSettings.Value.EtcdMembersUri;        // <<<<< тут NULL ?????????

        }

        public async Task<ResponseEtcd> Handle(RequestEtcd request, CancellationToken cancellationToken)
        {
            if (string.IsNullOrEmpty(_etcdMembersUri))
            {
                // todo: return error
            }
              .....
        }
    }
}