Hacky fix for mlflow logger (#277)

* Hacky fix for mlflow logger

It dies when "created_at" is logged

* Log warning

pytorch_lightning/logging/mlflow_logger.py

@@ -34,6 +34,11 @@ class MLFlowLogger(LightningLoggerBase):
     def log_metrics(self, metrics, step_num=None):
         timestamp_ms = int(time() * 1000)
         for k, v in metrics.items():
+            if isinstance(v, str):
+                logger.warning(
+                    f"Discarding metric with string value {k}={v}"
+                )
+                continue
             self.client.log_metric(self.run_id, k, v, timestamp_ms, step_num)
 
     def save(self):