Aggregates variables a keras metric placed at CLIENTS to SERVER.

metrics A single or a Sequence of tf.keras.metrics.Metric objects, or a single or a Sequence of no-arg callables that each constructs a tf.keras.metrics.Metric. The order must match the order of variables in federated_values.
federated_values A single federated value, or a Sequence of federated values. The values must all have tff.CLIENTS placement. If value is a Sequence type, it must match the order of the sequence in `metrics.

The result of performing a federated sum on federated_values, then assigning the aggregated values into the variables of the corresponding tf.keras.metrics.Metric and calling tf.keras.metrics.Metric.result. The resulting structure has tff.SERVER placement.