Fixing 6088 : Download CSV data (tensorflow#6719)

## Motivation for features / changes
Fixing 6088 : Download CSV data

## Technical description of changes
Setting good parameters to constructor and adding a default value for an
optional parameter

## Detailed steps to verify changes work correctly (as executed by you)
Download with no error

Googlers, see b/307370637. @yatbear tested internally at cl/598969320.

tensorboard/plugins/hparams/get_experiment.py

@@ -14,6 +14,8 @@
 # ==============================================================================
 """Classes and functions for handling the GetExperiment API call."""
 
+from tensorboard.plugins.hparams import api_pb2
+
 
 class Handler:
     """Handles a GetExperiment request."""
@@ -27,7 +29,7 @@ def __init__(
           request_context: A tensorboard.context.RequestContext.
           backend_context: A backend_context.Context instance.
           experiment_id: A string, as from `plugin_util.experiment_id`.
-          request: A api_pb2.GetExperimentRequest instance.
+          request: A request proto.
         """
         self._request_context = request_context
         self._backend_context = backend_context
@@ -38,7 +40,11 @@ def __init__(
             not request.HasField("include_metrics")
             or request.include_metrics
         )
-        self._hparams_limit = request.hparams_limit
+        self._hparams_limit = (
+            request.hparams_limit
+            if isinstance(request, api_pb2.GetExperimentRequest)
+            else None
+        )
 
     def run(self):
         """Handles the request specified on construction.

tensorboard/plugins/hparams/hparams_plugin.py

@@ -93,7 +93,7 @@ def download_data_route(self, request):
                 ctx, self._context, experiment_id, request_proto
             ).run()
             experiment = get_experiment.Handler(
-                ctx, self._context, experiment_id
+                ctx, self._context, experiment_id, request_proto
             ).run()
             body, mime_type = download_data.Handler(
                 self._context,