Messaging metrics: extending general messaging metrics for individual systems

[lmolkova]

Follow up on #163 (comment)

It makes sense for individual messaging systems to add system-specific attributes:

• Kafka consumers would want to report partition id
• RabbitMQ would report routing key
• Azure messaging systems also have partition or subscription that are important for metrics.

We should prototype/document custom metrics for 1+ messaging systems and document how extension can be done.

[pyohannes]

We should implement a solution according to the conclusions from #760.