protoc-gen-swagger: fix description of google/protobuf/struct.proto types

[johanbrandhorst]

The current swagger definition of google.protobuf.Struct looks like so:

{
  "fields": {
    "additionalProp1": {
      "null_value": "NULL_VALUE",
      "number_value": 0,
      "string_value": "string",
      "bool_value": true,
      "struct_value": {
        "fields": {
          "additionalProp1": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp2": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp3": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        }
      },
      "list_value": {
        "values": [
          {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        ]
      }
    },
    "additionalProp2": {
      "null_value": "NULL_VALUE",
      "number_value": 0,
      "string_value": "string",
      "bool_value": true,
      "struct_value": {
        "fields": {
          "additionalProp1": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp2": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp3": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        }
      },
      "list_value": {
        "values": [
          {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        ]
      }
    },
    "additionalProp3": {
      "null_value": "NULL_VALUE",
      "number_value": 0,
      "string_value": "string",
      "bool_value": true,
      "struct_value": {
        "fields": {
          "additionalProp1": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp2": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          },
          "additionalProp3": {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        }
      },
      "list_value": {
        "values": [
          {
            "null_value": "NULL_VALUE",
            "number_value": 0,
            "string_value": "string",
            "bool_value": true,
            "list_value": {
              "values": [
                null
              ]
            }
          }
        ]
      }
    }
  }
}

This is the structure of the protobuf message, not how it's parsed by the parser. It should be simple object, since it supports arbitray JSON.

Having briefly looked at the source (https://github.com/grpc-ecosystem/grpc-gateway/blob/master/protoc-gen-swagger/genswagger/template.go#L21), I think we're also missing the other struct.proto types, ListValue, Value and NullValue.