[Api-ref] Add metatdata parameters to backup create/update

Support metadata for backup resource was introduced in cinder [1]
but it's not documented in the API docs anywhere, This change is
to add them in the api docs.
[1] https://review.openstack.org/#/c/471541/

Change-Id: I0b444b9c48f156786433b7f622bae5dd6f274c59
Closes-bug: #1709254

api-ref/source/v3/ext-backups.inc

@@ -239,6 +239,7 @@ Request
    - name: name_optional
    - snapshot_id: snapshot_id_2
    - project_id: project_id_path
+   - metadata: metadata_9
 
 Request Example
 ---------------
@@ -277,7 +278,7 @@ Request
    - backup: backup
    - description: description
    - name: name_optional
-
+   - metadata: metadata_9
 
 Request Example
 ---------------

api-ref/source/v3/parameters.yaml

@@ -1343,6 +1343,13 @@ metadata_8:
   in: body
   required: true
   type: object
+metadata_9:
+  description: |
+    The backup metadata key value pairs.
+  in: body
+  required: false
+  type: object
+  min_version: 3.43
 migration_policy:
   description: |
     Specify if the volume should be migrated when it is re-typed.

api-ref/source/v3/samples/backup-create-request.json

@@ -4,6 +4,7 @@
         "description": null,
         "name": "backup001",
         "volume_id": "64f5d2fb-d836-4063-b7e2-544d5c1ff607",
-        "incremental": true
+        "incremental": true,
+        "metadata": null
     }
 }

api-ref/source/v3/samples/backup-create-response.json

@@ -11,6 +11,7 @@
                 "rel": "bookmark"
             }
         ],
-        "name": "backup001"
+        "name": "backup001",
+        "metadata": {}
     }
 }

api-ref/source/v3/samples/backup-update-request.json

@@ -1,6 +1,9 @@
 {
     "backup":{
         "name":"test",
+        "metadata": {
+            "key": "value"
+        },
         "description": "this is a backup"
     }
 }

api-ref/source/v3/samples/backup-update-response.json

@@ -5,12 +5,14 @@
             {
                 "href": "http://10.3.150.25:8776/v3/a7090a26bc554d93aa845a4d41808251/backups/fad41a83-203d-4998-9d3b-444fd5da5aba",
                 "rel": "self"
-      },
+            }, {
-      {
                 "href": "http://10.3.150.25:8776/a7090a26bc554d93aa845a4d41808251/backups/fad41a83-203d-4998-9d3b-444fd5da5aba",
                 "rel": "bookmark"
             }
         ],
-    "name": "test"
+        "name": "test",
+        "metadata": {
+            "key": "value"
+        }
     }
 }