[BUG] Prometheus parser 0.22.0 does not early skip comment lines

[wissamir]

Issue

Starting with version 0.22.0, comment lines do not skip validation and parsing early enough and third token goes into _is_valid_legacy_metric_name which results with an error when it does not match the METRIC_NAME_RE

Example:

"""# A random comment which goes into validation and passes"""
"""# Another random. comment which goes into validation and fails"""
"""# Whatever 335/125 random comment which goes into validation and fails"""

Expected behaviour:
Skip all comment lines. list(text_string_to_metric_families) should result with empty list []

Current behaviour:
A ValueError is raised for the second and third comment lines above

I'll be pushing a PR for the fix

[wissamir]

Fix proposal PR #1109