Support a legend key from metric or resource sources

Author: jharshman

tsplot/plot.go

@@ -105,12 +105,16 @@ func NewPlotFromTimeSeriesIterator(tsi *monitoring.TimeSeriesIterator, legendKey
 
 		// add to legend
 		if legendKey != "" {
-			legendEntry, _ := plotter.NewPolygon()
-			legendEntry.Color = lineColor
+			legendThumbnail, _ := plotter.NewPolygon()
+			legendThumbnail.Color = lineColor
 			p.Legend.Left = true
 			p.Legend.Top = true
 			p.Legend.Padding = vg.Points(2)
-			p.Legend.Add(timeSeries.GetMetric().GetLabels()[legendKey], legendEntry)
+			legendEntry := timeSeries.GetMetric().GetLabels()[legendKey]
+			if legendEntry == "" {
+				legendEntry = timeSeries.GetResource().GetLabels()[legendKey]
+			}
+			p.Legend.Add(legendEntry, legendThumbnail)
 		}
 	}