static long validateIdleTimeoutMillis(long idleTimeoutMillis) {
        if (idleTimeoutMillis < 0) {
            throw new IllegalArgumentException("idleTimeoutMillis: " + idleTimeoutMillis + " (expected: >= 0)");
        }
        return idleTimeoutMillis;
    }
Initialized later by {@link Server} via {@link #setServer(Server)}.
core/src/main/java/com/linecorp/armeria/server/DefaultServerConfig.java