Update scylla_io_queue_total_delay_sec metrics dispay (if possible)

[xemul]

After fixing the #1570 there appeared an inconvenience. The problem is that the ..._total_delay_sec metrics shows the sum of individual requests delays over some priod of time. And, for example, if there had passed 1k of requests with 50us delay each the metrics would report delta of 1k * 50us = 50ms. Thus when looking at the metrics it's always needed to make the reverse division (reported delay / reported IOPS) to get the idea of individual request latency.

If that's possible the dashboard should draw the rate(total_delay_sec) / rate(IOPS).

[amnonh]

@xemul is this only relevant to 5.0 and 2022.1 ?

[xemul]

No, the ..._total_delay_sec appeared earlier. It's exported in 4.5.3 and 4.5.6