expfmt should parse HELP text

[gouthamve]

See: prometheus/prometheus#4602

promtool is using expfmt https://github.com/prometheus/prometheus/blob/master/util/promlint/promlint.go#L68 which doesn't check the HELP text.

[brian-brazil]

No, the issue here is the ToUpper on https://github.com/prometheus/common/blob/master/expfmt/text_parse.go#L507