fix(monitoring): one value per stat panel

The OSD panel carried two queries, so it rendered two numbers side by side.
Both read the same figure in the normal case, which looks like a duplicate and
carries no information — a status box has one job, which is to be one colour.

It now shows OSDs down alone. An OSD that is up but out was the second number;
that is a state rather than a fault, Ceph already raises a health warning for
it, and it does not belong in a red-or-green box.

Refs #549

kubernetes/apps/monitoring/dashboards/disk-health.json

@@ -28,7 +28,7 @@
     },
     {
       "datasource": {"type": "prometheus", "uid": "prometheus"},
-      "description": "OSDs the cluster knows about but which are not up, and not in. Both should read zero. Counted as a difference against ceph_osd_metadata rather than compared to a fixed number, so adding or removing an OSD does not turn this panel into a false alarm.",
+      "description": "OSDs the cluster knows about but which are not up. Should read zero. Counted as a difference against ceph_osd_metadata rather than compared to a fixed number, so adding or removing an OSD does not turn this panel into a false alarm. An OSD that is up but out shows as a Ceph health warning instead — it is a state, not a fault, and it does not belong in a red-or-green box.",
       "fieldConfig": {
         "defaults": {
           "color": {"mode": "thresholds"},
@@ -42,10 +42,9 @@
       "id": 11,
       "options": {"colorMode": "background", "graphMode": "none", "justifyMode": "auto", "orientation": "auto", "reduceOptions": {"calcs": ["lastNotNull"], "fields": "", "values": false}, "textMode": "auto"},
       "targets": [
-        {"datasource": {"type": "prometheus", "uid": "prometheus"}, "expr": "sum(ceph_osd_metadata) - sum(ceph_osd_up)", "legendFormat": "down", "refId": "A"},
-        {"datasource": {"type": "prometheus", "uid": "prometheus"}, "expr": "sum(ceph_osd_metadata) - sum(ceph_osd_in)", "legendFormat": "out", "refId": "B"}
+        {"datasource": {"type": "prometheus", "uid": "prometheus"}, "expr": "sum(ceph_osd_metadata) - sum(ceph_osd_up)", "legendFormat": "", "refId": "A"}
       ],
-      "title": "OSDs down / out",
+      "title": "OSDs down",
       "type": "stat"
     },
     {