Integer overflow in bokeh update of system monitor

[fjetter]

It appears we get an integer overflow in our bokeh dashboard in tests like test_scheduler_bokeh.py::.test_simple, e.g. https://github.com/dask/distributed/actions/runs/3824315377/jobs/6506350808

Previous attempt to fix this #7426

[fjetter]

I believe these test failures are only on windows. afaik, windows is defaulting to a int32. that would very easy to hit with memory in bytes.

[mrocklin]

Ah! That makes sense. Good find:

Solutions could include:

1. Using int64 explicitly
2. Creating a sum function that casts first, or try-except

I've pushed up a simplistic fix in #7443