Choose a metric with high resolution as 'base' metric

This way if we discover the metrics have different resolutions, we'll
return the one containing (hopefully) the most information.  Note that
we still might have 'more' information (in terms of non-None data
points) if we chose a different metric instead.  Future work, I guess.

Updates issue #4.

main.go

@@ -390,6 +390,15 @@ func handleRenderPB(w http.ResponseWriter, req *http.Request, format string, res
 		return
 	}
 
+	// Use the metric with the highest resolution as our base
+	var highest int
+	for i, d := range decoded {
+		if d.GetStepTime() < decoded[highest].GetStepTime() {
+			highest = i
+		}
+	}
+	decoded[0], decoded[highest] = decoded[highest], decoded[0]
+
 	metric := decoded[0]
 
 	mergeValues(req, &metric, decoded)