public long readBytes() {
        synchronized (this) {
            long curMs = time.milliseconds();
            if (curMs != lastUpdateMs) {
                updateValues(curMs);
            }
            return cachedReadBytes;
        }
    }
Retrieves Linux /proc/self/io metrics.
server/src/main/java/org/apache/kafka/server/metrics/LinuxIoMetricsCollector.java