Well-known types reference

import "google/protobuf/any.proto";
import "google/protobuf/timestamp.proto";
// etc.

message Any {
  // Identifies the type of the serialized Protobuf message with a URI reference
  // consisting of a prefix ending in a slash and the fully-qualified type name.
  // Example: type.googleapis.com/google.protobuf.StringValue
  string type_url = 1;

  // Holds a Protobuf serialization of the type described by type_url.
  bytes value = 2;
}

// Wrapping a google.protobuf.Duration:
{
  "@type": "type.googleapis.com/google.protobuf.Duration",
  "value": "1.212s"
}

// Wrapping a custom message:
{
  "@type": "type.googleapis.com/mypackage.MyMessage",
  "fieldOne": "hello",
  "fieldTwo": 42
}

message Timestamp {
  // Seconds of UTC time since Unix epoch 1970-01-01T00:00:00Z.
  // Must be between -62135596800 and 253402300799 inclusive.
  int64 seconds = 1;

  // Non-negative fractions of a second at nanosecond resolution.
  // Must be between 0 and 999,999,999 inclusive.
  int32 nanos = 2;
}

"2017-01-15T01:30:15.01Z"

message Duration {
  // Signed seconds of the span of time.
  // Must be from -315,576,000,000 to +315,576,000,000 inclusive.
  int64 seconds = 1;

  // Signed fractions of a second at nanosecond resolution.
  // Must be from -999,999,999 to +999,999,999 inclusive.
  // The sign of nanos must match seconds for non-zero values.
  int32 nanos = 2;
}

"3s"          // 3 seconds exactly
"3.000000001s" // 3 seconds and 1 nanosecond
"3.000001s"   // 3 seconds and 1 microsecond

// Represents a JSON object: an unordered map of string keys to Values.
message Struct {
  map<string, Value> fields = 1;
}

// Represents a JSON value (null, number, string, bool, object, or array).
message Value {
  oneof kind {
    NullValue null_value  = 1;
    double    number_value = 2;
    string    string_value = 3;
    bool      bool_value   = 4;
    Struct    struct_value = 5;
    ListValue list_value   = 6;
  }
}

// Sentinel for JSON null.
enum NullValue {
  NULL_VALUE = 0;
}

// Represents a JSON array.
message ListValue {
  repeated Value values = 1;
}

message Empty {}

// Typical usage in a service:
service Foo {
  rpc Bar(google.protobuf.Empty) returns (google.protobuf.Empty);
}

message FieldMask {
  // The set of field mask paths.
  repeated string paths = 1;
}

// Proto text format:
// paths: "user.display_name"
// paths: "photo"

// JSON:
"user.displayName,photo"

message DoubleValue  { double  value = 1; }
message FloatValue   { float   value = 1; }
message Int64Value   { int64   value = 1; }
message UInt64Value  { uint64  value = 1; }
message Int32Value   { int32   value = 1; }
message UInt32Value  { uint32  value = 1; }
message BoolValue    { bool    value = 1; }
message StringValue  { string  value = 1; }
message BytesValue   { bytes   value = 1; }