The `_created` auto-created metrics (e.g. for Counter) cause failure of `promtool check metrics` because of lack of HELP

[davidlukac]

• prometheus/client_python version: 0.7.1
• Python version 3.7.1
• promtool version from current HEAD: 2.13.0-rc.0

When creating a counter, e.g.

c = Counter('some_counter', "Some counter")

this will create two metrics:

# HELP some_counter Some counter
# TYPE some_counter counter
some_counter_total 4.0
# TYPE some_counter_created gauge
some_counter_created 1.5699349744874952e+09

The output lacks the HELP line for the _created metric. This causes a failure with promtool:

$ curl -s http://localhost:8080/metrics | promtool check metrics
some_counter_created no help text

[davidlukac]

Resolved in PR #471