GRR Client Crashes panel (in AdminUI dashboard) does not work as expected

As mentioned in #787, the [GRR Client Crashes panel](https://github.com/tsehori/grr/blob/b8cd73652f9251f54fe059cebc42ca26bd601184/monitoring/grafana/grr_grafanalib_dashboards/reusable_panels.py#L103), which is a part of the [AdminUI dashboard](https://github.com/tsehori/grr/blob/b8cd73652f9251f54fe059cebc42ca26bd601184/monitoring/grafana/grr_grafanalib_dashboards/dashboards_to_generate/admin_ui.dashboard.py), does the following: when GRR clients are not crashing, the graph shows a flat line. The problem is that, even when GRR clients do crash, the graph remains as a flat line, where we expect to see a "spike" that indicates that something goes wrong.
I suspect that the issue is not with the query implemented in #787, but that the client crash is not recorded well in the [Prometheus Stats collector](https://github.com/google/grr/blob/b2969c7ed6f5c6e62919fa66625d992e7b5e0b34/grr/server/grr_response_server/prometheus_stats_collector.py#L149).