It has
https://apis-explorer.appspot.com/apis-explorer/?base=https://chromium-swarm.appspot.com/_ah/api#p/swarming/v1/swarming.task.result

It's a flag to pass as a GET query (since this request is ultimately cacheable when the task is completed).

Here's an example:
https://apis-explorer.appspot.com/apis-explorer/?base=https://chromium-swarm.appspot.com/_ah/api#p/swarming/v1/swarming.task.result?task_id=2ebe1ebf3b15e010&include_performance_stats=true&_h=2&

There's no flag in the client tool to specify this. Is that the request here?

Great!  So yes, it looks like the client just needs to be updated to add the flag, eg. https://github.com/luci/luci-go/blob/master/common/api/swarming/swarming/v1/swarming-gen.go#L2455

Yes, I was waiting for https://codereview.chromium.org/1946253003/ to be committed first be you can regenerate the files right away with //tools/cmd/apigen.

The following revision refers to this bug:
  https://chromium.googlesource.com/external/github.com/luci/luci-go.git/+/b739988e74e12a7abbd6a1db13e7a09f50b62424

commit b739988e74e12a7abbd6a1db13e7a09f50b62424
Author: borenet <borenet@chromium.org>
Date: Fri May 20 19:34:11 2016

Re-generate Swarming API Go client

BUG= 611149 

Review-Url: https://codereview.chromium.org/2000803002

[modify] https://crrev.com/b739988e74e12a7abbd6a1db13e7a09f50b62424/common/api/swarming/swarming/v1/swarming-api.json
[modify] https://crrev.com/b739988e74e12a7abbd6a1db13e7a09f50b62424/common/api/swarming/swarming/v1/swarming-gen.go