code: @SuppressWarnings("WeakerAccess")
    public CreateInstanceRequest addLabel(@Nonnull String key, @Nonnull String value) {
      Preconditions.checkNotNull(key, "Key can't be null");
      Preconditions.checkNotNull(value, "Value can't be null");
      builder.getInstanceBuilder().putLabels(key, value);
      return this;
    }
docs: Adds an arbitrary label to the instance.
  
  <p>Labels are key-value pairs that you can use to group related instances and store metadata
  about an instance.
  
  @see <a href="https://cloud.google.com/bigtable/docs/creating-managing-labels">For more
  details</a>