docs: def measurement_key(
          val: Any,
          default: Any = RaiseTypeErrorIfNotProvided):
      """Get the measurement key for the given value.
  
      Args:
          val: The value which has the measurement key..
          default: Determines the fallback behavior when `val` doesn't have
              a measurement key. If `default` is not set, a TypeError is raised.
              If default is set to a value, that value is returned if the value
              does not have `_measurement_key_`.
  
      Returns:
          If `val` has a `_measurement_key_` method and its result is not
          `NotImplemented`, that result is returned. Otherwise, if a default
          value was specified, the default value is returned.
  
      Raises:
          TypeError: `val` doesn't have a _measurement_key_ method (or that method
              returned NotImplemented) and also no default value was specified.
      """
      getter = getattr(val, '_measurement_key_', None)
      result = NotImplemented if getter is None else getter()
      if result is not NotImplemented:
          return result
  
      if default is not RaiseTypeErrorIfNotProvided:
          return default
  
      if getter is None:
          raise TypeError(
                  "object of type '{}' has no _measurement_key_ method."
                      .format(type(val)))
  
      raise TypeError("object of type '{}' does have a _measurement_key_ method, "
                      "but it returned NotImplemented.".format(type(val)))