Marked dashboard_json field as a json field (#14818) (#10521)

[upstream:8f3609e321e1c9eaac8692d0ed40cefb77f26883]

Signed-off-by: Modular Magician <magic-modules@google.com>

Author: modular-magician

google-beta/services/monitoring/resource_monitoring_dashboard_meta.yaml

@@ -5,4 +5,5 @@ api_version: 'v1'
 api_resource_type_kind: 'Dashboard'
 fields:
   - field: 'dashboard_json'
+    json: true
   - field: 'project'