// marshaling to YAML and JSON. In particular, it marshals into strings, which
// can be used as map keys in json.
message Duration {
  optional int64 duration = 1;
}

// FieldsV1 stores a set of fields in a data structure like a Trie, in JSON format.
//
// Each key is either a '.' representing the field itself, and will always map to an empty set,