public void AddCustomField()
{
var tId = Guid.NewGuid().ToString();
var ctx = new CorrelationContext(tId);
Assert.Throws <ArgumentException>(() => ctx.Add("correlationId", "id"));
Assert.Throws <ArgumentException>(() => ctx.Add("requestId", "id"));
ctx.Add("custom", 123);
Assert.Equal(123, ctx["custom"]);
Assert.Equal(3, ctx.Count);
}