Add schema for dashboard custom templating variable

Update sample dashboard fixtures

spec/fixtures/lib/gitlab/metrics/dashboard/sample_dashboard.yml

@@ -13,6 +13,17 @@ templating:
       options:
         default_value: 'default'
     text_variable_simple_syntax: 'default value'
+    custom_variable_simple_syntax: ['value1', 'value2', 'value3']
+    custom_variable_full_syntax:
+      label: 'Variable 2'
+      type: custom
+      options:
+        values:
+        - value: 'value option 1'
+          text: 'Option 1'
+        - value: 'value_option_2'
+          text: 'Option 2'
+          default: true
 panel_groups:
 - group: Group A
   priority: 1

spec/fixtures/lib/gitlab/metrics/dashboard/schemas/custom_variable_full_syntax.json

+{
+  "type": "object",
+  "required": [
+    "type", "options"
+  ],
+  "properties": {
+    "type": { "enum": ["custom"] },
+    "label": { "type": "string" },
+    "options": { "$ref": "custom_variable_options.json" }
+  },
+  "additionalProperties": false
+}

spec/fixtures/lib/gitlab/metrics/dashboard/schemas/custom_variable_options.json

+{
+  "type": "object",
+  "required": ["values"],
+  "properties": {
+    "values": {
+      "type": "array",
+      "items": { "$ref": "custom_variable_values.json" }
+    }
+  },
+  "additionalProperties": false
+}

spec/fixtures/lib/gitlab/metrics/dashboard/schemas/custom_variable_values.json

+{
+  "type": "object",
+  "required": ["value"],
+  "properties": {
+    "value": { "type": "string" },
+    "text": { "type": "string" },
+    "default": { "type": "boolean" }
+  },
+  "additionalProperties": false
+}

spec/fixtures/lib/gitlab/metrics/dashboard/schemas/text_variable_full_syntax.json

@@ -4,7 +4,7 @@
     "type", "options"
   ],
   "properties": {
-    "type": { "type": "string" },
+    "type": { "enum": ["text"] },
     "label": { "type": "string" },
     "options": { "$ref": "text_variable_options.json" }
   },

spec/fixtures/lib/gitlab/metrics/dashboard/schemas/variables.json

@@ -4,7 +4,12 @@
     "^[a-zA-Z0-9_]*$": {
         "anyOf": [
           { "$ref": "text_variable_full_syntax.json" },
-          { "type": "string" }
+          { "type": "string" },
+          {
+            "type": "array",
+            "items": { "type": "string" }
+          },
+          { "$ref": "custom_variable_full_syntax.json" }
         ]
       }
   },