promtool check metrics does not validate uniqueness

[carlpett]

Bug Report

What did you do?
curl -s ... | promtool check metrics

What did you expect to see?
According to the specs:

Each line must have a unique combination of a metric name and labels. Otherwise, the ingestion behavior is undefined.

We had an endpoint that served duplicates for some name/label sets, but promtool does not complain.

What did you see instead? Under which circumstances?
Nothing

Environment
promtool from master/c8a1a5a9

An independent repro without our faulty exporter is

# HELP foo testing
# TYPE counter
foo{bar="1"} 1
foo{bar="1"} 2
EOF

• System information:
  Darwin 18.2.0 x86_64

• Prometheus version:
  git c8a1a5a

[geekodour]

will send the fix if I can reproduce the issue on master.