Add GPUCurrentLoad dashboard plots

[mrocklin]

A first step towards fixing rapidsai/dask-cuda#36

[pentschev]

A bit of an unrelated question, but I thought you wanted to keep GPU-specific code mostly out of dask/distributed. This is just for my own understanding, what's the current policy on whether GPU code should go in here rather than dask-cuda?

[mrocklin]

It's a good question. I could imagine moving this around later in the future.

There was some non-trivial amount of work to make the dashboarding general enough to take information like this. I was also able to find a way to keep things nicely separate. I also have decent confidence that this code won't need much experimentation in the future, and will be low churn.

[mrocklin]

For example, we defined cuda_serialize in distributed because the code changes needed for it are mostly the same as the code changes needed for dask_serialize. The infrastructure is there. Things like the dask-cuda-worker code is changing rapidly enough that I'd prefer to keep it external for a while. I could imagine moving it in once it's more stable.

[pentschev]

Ok, makes sense. Thanks for the clarifications @mrocklin .

Now going back to this PR, would you like me to test it out and review it?

[mrocklin]

Review would be welcome

[pentschev]

It looks good to me, I added a couple of minor comments. I don't have experience with bokeh, superficially it looks fine for me, but maybe someone else with bokeh experience could take a look at that, otherwise, good to be merged by me.

[pentschev]

LGTM, looking forward to see this being used by more people!