## Bug description

When going to instance status > internal metrics and refreshing the dashboard, the loading fails

https://user-images.githubusercontent.com/148820/120655500-693c7480-c48b-11eb-8c75-6afd4c967506.mp4

## Expected behavior

It reloads successfully

## How to reproduce

See above

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: latest

## Additional context

Seems the cause is that "refresh" requests data from `/api/insight/trends` vs getting it from `/api/dashboard/id` as on first load.

Relates to #3609  

#### *Thank you* for your bug report – we love squashing them!

posthog/api/dashboard.py
request = self.context["request"]
        team = Team.objects.get(id=self.context["team_id"])
        validated_data.pop("last_refresh", None)  
        if not validated_data.get("dashboard", None):
            dashboard_item = DashboardItem.objects.create(team=team, created_by=request.user, **validated_data)
            return dashboard_item
        elif validated_data["dashboard"].team == team:
            created_by = validated_data.pop("created_by", request.user)
            dashboard_item = DashboardItem.objects.create(
                team=team, last_refresh=now(), created_by=created_by, **validated_data
            )
            return dashboard_item
        else:
            raise serializers.ValidationError("Dashboard not found")
    def update(self, instance: Model, validated_data: Dict, **kwargs) -> DashboardItem:
        
        validated_data.setdefault("is_sample", False)
        return super().update(instance, validated_data)
    def get_result(self, dashboard_item: DashboardItem):
        
        if dashboard_item.last_refresh and (now() - dashboard_item.last_refresh).days > 0:
            return None
        if not dashboard_item.filters_hash:
            return None
        result = get_safe_cache(dashboard_item.filters_hash)
        if not result or result.get("task_id", None):
            return None
        return result.get("result")
    def get_last_refresh(self, dashboard_item: DashboardItem):