Fix up endTimestamp to be Monarch compliant

[pmarkowsky]

This PR fixes an issue with our Monarch JSON that causes some metrics to be wonky.

Specifically our endTimestamp field needs to be the time of the export not the last_updated time.

[mlw]

How I'm reading this - it appears like this will make the last_updated values reported by SNTCommandMetrics not match what is sent to the metrics server - is that desireable?

[pmarkowsky]

@mlw Short answer is yes. Monarch requires the time series to be reported constantly so as long as we have the value we have to record a new endTimestamp, not the last time it was actually updated.

If we want last_updated to be the last time the metric changed then we need to do this differently if we just want everything to be the last time we exported then we can simplify the SNTMetricSet to just always add the timestamp on export.