Setting dimensions as null breaks metrics logger

[Glen-Moonpig]

I had this code:

class MyMetricsLogger
{
    private readonly MetricsLogger _metricsLogger = new MetricsLogger();
    public void LogMetric(string key, double value, DimensionSet dimensionSet = null)
    {
        _metricsLogger.SetDimensions(dimensionSet);
        _metricsLogger.PutMetric(key, value);
        _metricsLogger.Flush();
    }
}

Invoking MetricsLogger.SetDimensions with null will break the MetricsLogger. When it next tries to log any metrics there is a NullReferenceException thrown from

   at Amazon.CloudWatch.EMF.Model.MetricDirective.<>c.<get_AllDimensionKeys>b__20_0(DimensionSet s)

There is a simple workaround, avoid using null when calling SetDimensions.

if (dimensionSet != null)
{
    _metricsLogger.SetDimensions(dimensionSet);
}
else
{
    _metricsLogger.SetDimensions();
}

It is a weakness of the package design for me to have to do this, the null check should be handled gracefully within the package itself.

[markkuhn]

Fixed by #37