fix(telemetry): show the network type in the FullBelowCache gauge title

Single-value panels carry the filtered network type in their title because
they have no legend to put it in; multi-series panels carry it in
legendFormat instead. This gauge was the only single-value panel across the
fifteen dashboards without it, so its reading was ambiguous once more than
one network type was in scope.

docker/telemetry/grafana/dashboards/node-health.json

@@ -3017,7 +3017,7 @@
               "refId": "A"
             }
           ],
-          "title": "FullBelowCache Hit Rate",
+          "title": "FullBelowCache Hit Rate [$xrpl_network_type]",
           "type": "gauge"
         }
       ],