NPE in `AbstractMetricsCollector` when channel limit is exceeded

[Dav1dde]

ChannelManager#createChannel can return null when it runs out of free channels, the JavaDoc of AMQConnection#createChannel also says it may return null.

The following code in createChannel() and createChannel(int channelNumber)

rabbitmq-java-client/src/main/java/com/rabbitmq/client/impl/AMQConnection.java

Lines 598 to 600 in 4142f8d

	Channel channel = cm.createChannel(this, channelNumber);
	metricsCollector.newChannel(channel);
	return channel;

leads to an NPE in AbstractMetricsCollector#newChannel:

rabbitmq-java-client/src/main/java/com/rabbitmq/client/impl/AbstractMetricsCollector.java

Lines 85 to 87 in 4142f8d

	incrementChannelCount(channel);
	channel.addShutdownListener(cause -> closeChannel(channel));
	connectionState(channel.getConnection()).channelState.put(channel.getChannelNumber(), new ChannelState(channel));

channel.addShutdownListener will fail with the NPE.

We've observed this through logs in one of our production services:

01-25 12:52:30 INFO  c.r.c.impl.AbstractMetricsCollector - Error while computing metrics in newChannel: Cannot invoke "com.rabbitmq.client.Channel.addShutdownListener(com.rabbitmq.client.ShutdownListener)" because "channel" is null