[bugfix] 'No numeric types to aggregate' in NVD3 chart (apache#5853)

(cherry picked from commit 73cfccc)

superset/viz.py

@@ -227,15 +227,15 @@ def get_df(self, query_obj=None):
                 df[DTTM_ALIAS] += self.time_shift
 
             if self.enforce_numerical_metrics:
-                self.df_metrics_to_num(df, query_obj.get('metrics') or [])
+                self.df_metrics_to_num(df)
 
             df.replace([np.inf, -np.inf], np.nan)
             df = self.handle_nulls(df)
         return df
 
-    @staticmethod
-    def df_metrics_to_num(df, metrics):
+    def df_metrics_to_num(self, df):
         """Converting metrics to numeric when pandas.read_sql cannot"""
+        metrics = self.metric_labels
         for col, dtype in df.dtypes.items():
             if dtype.type == np.object_ and col in metrics:
                 df[col] = pd.to_numeric(df[col], errors='coerce')