Parse error on empty HELP

[Wing924]

Bug Report

What did you do?
Scrape those metrics:

# HELP rabbitmq_channels  
# TYPE rabbitmq_channels gauge
rabbitmq_channels{name="rabbit",} 0.0
# HELP rabbitmq_published_total  
# TYPE rabbitmq_published_total counter
rabbitmq_published_total{name="rabbit",} 0.0

What did you expect to see?
Parse OK

What did you see instead? Under which circumstances?
Error: expected text in HELP

Environment

• Prometheus version:
  • v2.3.2: OK
  • master: NG

Reason

textparse has been changed.
Previous one just ignore lines starting with # while new one parse comment lines.

prometheus/pkg/textparse/parse.go

Lines 275 to 287 in ffb7836

	case tHelp, tType:
	switch t := p.nextToken(); t {
	case tMName:
	p.offsets = append(p.offsets, p.l.start, p.l.i)
	default:
	return EntryInvalid, parseError("expected metric name after HELP", t)
	}
	switch t := p.nextToken(); t {
	case tText:
	p.text = p.l.buf()[1:]
	default:
	return EntryInvalid, parseError("expected text in HELP", t)
	}

These code don't allow help is empty.
It breaks the backward compatibility

[lock]

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