expose broker-level MQTT metrics

since we switched from v2 CloudFormation installer to v3 terraform [including the switch from AWS SQS to built-in MQTT], we have lost:

• having CloudWatch dashboards out of the box [had to terraform it myself]

• no MQTT-related metrics are in CloudWatch

Now without having the need to enable telemetry, I would like to request to add broker-level MQTT metrics to CloudWatch.

https://docs.spacelift.io/self-hosted/latest/installing-spacelift/reference-architecture/guides/observability.html#message-queues