static long nextPortGroup() {
        for (;;) {
            final long portGroup = nextPortGroup.incrementAndGet();
            if (portGroup > 0) {
                return portGroup;
            } else {
                // 0 means 'no group'.
            }
        }
    }
Returns a unique value that is used for identifying a group of {@link ServerPort}s.
When two ephemeral {@link ServerPort}s have the same port group value, {@link Server}
will choose the same port number for them, rather than allocating two ephemeral ports.
core/src/main/java/com/linecorp/armeria/server/ServerPort.java