NIFI-6111: Fixed bugs in the Status History values. If metrics are no…

[markap14]

…t available yet from all nodes for the last point on the graph, leave the cluster aggregate value off for the last point to prevent it from dropping significantly at the end of the chart. Fixed bug where counter values were not properly summed together in cluster view. Addressed issue with Average Task Duration

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[bbende]

Looks good, will merge