public CustomMetricsPoster(string apiKey, string instanceId = null, IFailureCallback failureCallback = null)
        {
            if (String.IsNullOrWhiteSpace(apiKey))
            {
                throw new ArgumentNullException("apiKey");
            }

            _apiKey = apiKey;
            _failureCallback = failureCallback ?? new ConsoleFailureCallback();
            InstanceId = instanceId;
        }
        public CustomMetricsPoster(string apiKey, string instanceId = null, IFailureCallback failureCallback = null)
        {
            if (String.IsNullOrWhiteSpace(apiKey))
            {
                throw new ArgumentNullException("apiKey");
            }

            _apiKey          = apiKey;
            _failureCallback = failureCallback ?? new ConsoleFailureCallback();
            InstanceId       = instanceId;
        }