public MetricCollectingRpcClient build(RpcClient delegate) {
        requireNonNull(delegate, "delegate");
        return new MetricCollectingRpcClient(delegate, meterIdPrefixFunction(), successFunction(),
                                             distributionStatisticConfig());
    }
Returns a newly-created {@link MetricCollectingRpcClient} decorating {@link RpcClient} based
on the properties of this builder.
core/src/main/java/com/linecorp/armeria/client/metric/MetricCollectingRpcClientBuilder.java