feature(app): Add a debugging summary function, exposed via http

[bboreham]

When someone says they have a performance problem, the first thing I want to know is whether Scope is reporting 26,000 processes on a node, or similar. It's awkward to do that via the raw reports, so I wrote this summarising function you can call via URL /admin/summary

Example output:

pod:183, service:81, deployment:87, daemon_set:8, stateful_set:2, cron_job:5, namespace:14, persistent_volume:3, persistent_volume_claim:4, storage_class:1
ip-172-20-2-197;<host>:endpoint:992, process:369, container:101, container_image:41, host:1, overlay:1
ip-172-20-1-59;<host>:endpoint:266, process:250, container:43, container_image:15, host:1, overlay:1
ip-172-20-3-247;<host>:endpoint:891, process:383, container:107, container_image:43, host:1, overlay:1

[fbarl]

Very nice, left only some minor comments! 👌

One more comment I have is that it would be nice to update some docs with this change, e.g.

• Add a sentence to a (new) Report Summary section in https://github.com/weaveworks/scope/blob/master/site/faq.md
• Update the Github issue template to paste the output of /admin/summary whenever filing a new bug

[bboreham]

Update the Github issue template to paste the output of /admin/summary whenever filing a new bug

I think I will leave this until experience tells us we need the info very often.