Split block_queue_length Prometheus metric into three

[AnnaShaleva]

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

We have three different block queues (the third is introduced in #3515). These queues share a single block_queue_length metric, and even though these queues doesn't work concurrently, I think that each of them deserves a separate metric.

Describe the solution you'd like

Split block_queue_length into three different metrics and use them for network server's block queues separately.

Describe alternatives you've considered

Leave it as is.