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