e2e: check labels documentation

[olivierlemasle]

As @lilic pointed out recently, CI checks thats every metric is documented (make doccheck), but there's no check on labels documentation.

That led to PR where labels documentation was not updated and CI did not catch this (#996).

Actually, it's more difficult to check labels:

• metric names are extracted from source code by a static analysis (with regex "(kube_[^"]+)"); labels are not easily found by basic static analysis;
• some labels have a dynamic name (e.g. label_JOB_LABEL)

What do you think of using e2e to check labels documentation? We already check metrics from /metrics endpoint; we could also list labels for each metric and check if they're in documentation (even if it is not exhaustive).

[brancz]

I think this is a great idea!

[olivierlemasle]

/assign