Major Description
BlockingEnvelopeMap currently hard-codes the metric group as this.getClass().getName(). This is generally OK, but there are occasions where we might want to override it (KafkaSystemConsumer being one of them).
I think we should update the BlockingEnvelopeMap to take a parameter for the metrics name, and update KafkaSystemConsumer to use classOf[KafkaSystemConsumerMetrics].getName().
