json_schema: {
      "$schema": "http://json-schema.org/draft-04/schema#",
      "description": "An array of key-value pairs that represent Apple HealthKit metadata information.",
      "type": "array",
      "references": [
          {
              "description": "A general description of metadata in HealthKit.",
              "url": "https://developer.apple.com/library/ios/documentation/HealthKit/Reference/HKObject_Class/index.html#//apple_ref/occ/instp/HKObject/metadata"
          }
      ],
      "items": {
          "type": "object",
          "properties": {
              "key": {
                  "description": "A predefined or custom metadata key. Use Apple defined keys, e.g. HKMetadataKeyDeviceSerialNumber, when possible.",
                  "type": "string",
                  "references": [
                      {
                          "description": "The list of predefined metadata keys in HealthKit.",
                          "url": "https://developer.apple.com/library/ios/documentation/HealthKit/Reference/HealthKit_Constants/#//apple_ref/doc/constant_group/Metadata_Keys"
                      }
                  ]
              },
              "value": {
                  "oneOf": [
                      {
                          "type": "string"
                      },
                      {
                          "type": "number"
                      },
                      {
                          "type": "boolean"
                      }
                  ]
              }
          },
          "required": [
              "key",
              "value"
          ]
      }
  }