Allow selecting specific CPUs on OS dahboard

[vladzcloudius]

System information

• Scylla version (you are using): unrelated
• Are you willing to contribute it (Yes/No): No

Describe the feature and the current behavior/state.
Today the "CPU used" graph on the "OS Metrics" dashboard looks like this:

There are two problems:

1. The by value is probably still shard in the example above. However the node_cpu_xxx metrics use a cpu label.
2. This dashboard doesn't even have a drop-down to select specific CPU. So even if (1) would be fixed there would be impossible to do something about it. ;)

Who will benefit with this feature?
Everybody.
In particular this would allow looking at IRQ CPUs utilization. Me personally and @avikivity are very interested in this specific use case.

[amnonh]

there's a mixed of unrelated things here, there's no shard/cpu level at the os dashboard, so someone add that manually, if tinstead of adding instance,shard they would have added instance,cpu it would have worked.
So that's not a limitation.
An example: