Skip metadata lines in dnsmasq leases

[cnaples79]

Summary

• ignore metadata lines such as duid and vendorclass when parsing the leases file
• return a sentinel error so the collector quietly skips these records instead of logging parse failures
• add regression coverage and sample data to demonstrate that metadata lines are skipped while lease metrics remain unchanged

Fixes #33

[ferrarimarco]

Thanks for this PR. Looping @stapelberg in for an additional review.