fix(telemetry): give the GetObject heatmap the options its plugin requires

The GetObject Request Size Distribution heatmap, added on this branch, was
missing calculate, color and cellGap. Grafana's heatmap plugin needs them, and
without them the whole dashboard opens with "An error occurred within the
plugin" instead of rendering.

Same values as the other four heatmaps fixed upstream, taken from the one that
renders correctly on Grafana Cloud. The panel keeps its own axis label and unit.

docker/telemetry/grafana/dashboards/overlay-traffic-detail.json

@@ -713,6 +713,16 @@
         "y": 41
       },
       "options": {
+        "calculate": false,
+        "cellGap": 1,
+        "color": {
+          "mode": "scheme",
+          "scheme": "Turbo",
+          "steps": 64
+        },
+        "legend": {
+          "show": true
+        },
         "tooltip": {
           "maxHeight": 600,
           "mode": "multi",