Make it easier to detect blocked connections

[jjank]

Is your feature request related to a problem? Please describe.

TL;DR: Detecting blocked connections can be quite tricky when you don't have access to the broker

• In the project I'm currently on we are using the rabbitmq-jms client (not my choice but we cannot change this in the forseeable future)
• For reasons beyond our control the RabbitMQ broker blocked our clients again and again (Ops is working to resolve this properly)
• We found it quite hard to get to the bottom of this, specifically:
  • We found no (supported?) way to add a BlockedListener using the rabbitmq-jms apis (as far as I remember access to com.rabbitmq.client.Connection is hidden behind the ConnectionFactory but that's a deliberate decision I suppose and a different discussion)
  • To the best of our research no default (e.g. logging) happens upon blocking
  • MetricsCollector does not record blocked connections

Describe the solution you'd like

1. Track/Record blocked connections in MetricsCollector - I've drafted a proposal and would be happy to file a PR for this to get your opinion
2. Optionally: Consider adding a default BlockedListener to AMQConnection that at the very least logs a warning upon blocking

Describe alternatives you've considered

Additional context