Allow google.protobuf.Timestamp in shipper Event fields and metadata

[cmacknz]

The shipper Event payload are currently defined as google.protobuf.Struct which require encoding timestamps as strings:

elastic-agent-shipper/api/shipper.proto

Lines 48 to 54 in 504c1e1

	// Metadata JSON object (map[string]google.protobuf.Value)
	google.protobuf.Struct metadata = 5;
	// Field JSON object (map[string]google.protobuf.Value)
	google.protobuf.Struct fields = 6;
	// Note: The google.protobuf.Value type should be extended (or re-implemented) with a
	// google.protobuf.Timestamp to avoid the cost of encoding timestamps as strings and then
	// parsing them back into a date representation.

Following from the note in the event definition, we should extend the google.protobuf.Struct type to allow using google.protobuf.Timestamp as one of the Value cases. The agent always writes to datastreams which require a timestamp, forcing timestamps to be encoded causes us to inefficiently parse them from string for every message received.

It should be enough to clone the existing google.protobuf.Struct definition and add the new message case in the Value oneof type, for example:

message Value {
  // The kind of value.
  oneof kind {
    // Represents a null value.
    NullValue null_value = 1;
    // Represents a double value.
    double number_value = 2;
    // Represents a string value.
    string string_value = 3;
    // Represents a boolean value.
    bool bool_value = 4;
    // Represents a structured value.
    Struct struct_value = 5;
    // Represents a repeated `Value`.
    ListValue list_value = 6;
    // Represents a timestamp.
    google.protobuf.Timestamp timestamp = 7;
  }
}