Update RabbitMQ-Overview.json - Disk space available before publishers blocked (#15723)

This change helps visualise the headroom (the gap before blocking) rather than just the raw available space. The updated query is now in line with the title that reads "Disk space available before publishers blocked".

Author: FSociety07

deps/rabbitmq_prometheus/docker/grafana/dashboards/RabbitMQ-Overview.json

@@ -1800,7 +1800,7 @@
             "uid": "${DS_PROMETHEUS}"
           },
           "editorMode": "code",
-          "expr": "rabbitmq_disk_space_available_bytes * on(instance, job) group_left(rabbitmq_cluster, rabbitmq_node) rabbitmq_identity_info{rabbitmq_cluster=\"$rabbitmq_cluster\", namespace=\"$namespace\", rabbitmq_endpoint=\"$endpoint\"}",
+          "expr": "(rabbitmq_disk_space_available_bytes - rabbitmq_disk_space_available_limit_bytes) * on(instance,job) group_left(rabbitmq_cluster, rabbitmq_node) rabbitmq_identity_info{rabbitmq_cluster=\"$rabbitmq_cluster\",namespace=\"$namespace\", rabbitmq_endpoint=\"$endpoint\"}",
           "format": "time_series",
           "instant": false,
           "intervalFactor": 1,