Might be possible to do like this?
https://chromium.googlesource.com/chromium/tools/build.git/+/master/scripts/slave/recipe_modules/ios/api.py#15

i.e.
1. make diagnose_goma to output JSON file that has summary of the step.
2. edit step result with the JSON output. 

What do you expect to be shown there?
I thought something like this:

diagnose_goma diagnose_goma 
goma: 29addd19aec04f3839bfcad8c853f858a3a91ff6
fatal: 1
error: 10
warnings: 200

I think we want to see summary about goma revision, version + why diagnose_goma reported failure. i.e. messages.

note that "SLOW Task:x time to start: d" is included in messages now, but it is usually too many. it would be better not to include messages but to show them at the end, but put just
"too many slow tasks to start: num, longest-duration-in-pending" or something in messages.

Messages sounds too much.

I prefer to show number of Fatal, Error, Warning in the second line, show non-repeated version of messages up to 3 lines (if more than 3, show ... in 4th line).

It is like:
goma: 29addd19aec04f3839bfcad8c853f858a3a91ff6
Fatal 1 > 0, error 30 > 10, warning 129 > 100
Too many slow tasks to start: 128, longest: 08.21 sec
Consumed too much memory: 3422429184 > 3221225472
Too much missing files: 1677 > 1024
...

What do you think?

ok.
messages is not so much different from this, except slow task lines.
actual fatal, errors, warnings are not in messages. 

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/goma/client/+/95564066b60b565c16b3558a125adbbcb7fdc81e

commit 95564066b60b565c16b3558a125adbbcb7fdc81e
Author: Yoshisato Yanagisawa <yyanagisawa@google.com>
Date: Thu Mar 24 09:47:20 2016

This issue is blocked by https://codereview.chromium.org/2020603002/.
The implementation is already in https://codereview.chromium.org/1870953002/ but we cannot use this without the former one.

We are suggested to do this in internal buildbot, and obtaining internal bot might take long time.  Let me close both change list and comes back when internal bot get ready, or we changed our mind to implement the other things to do the similar thing we wanted with this issue.

we run diagnose_goma_log_server to run diagnose goma log, so no need to do this.