message: fix metrics type error in pivot table viz
  transfer metrics dict label to list of string
diff: @@ -691,7 +691,7 @@ class PivotTableViz(BaseViz):
  df = df.pivot_table(
  index=self.form_data.get('groupby'),
  columns=self.form_data.get('columns'),
  - values=self.form_data.get('metrics'),
  + values=[self.get_metric_label(m) for m in self.form_data.get('metrics')],
  aggfunc=self.form_data.get('pandas_aggfunc'),
  margins=self.form_data.get('pivot_margins'),
  )
  