@Override
    public final boolean isClosing() {
        return pingState == PingState.SHUTDOWN;
    }
An {@link AbstractKeepAliveHandler} that writes a PING when neither read nor write was performed for
the specified {@code pingIntervalMillis}, and closes the connection
when neither read nor write was performed within the given {@code idleTimeoutMillis}.
core/src/main/java/com/linecorp/armeria/internal/common/AbstractKeepAliveHandler.java