fix: tolerate legacy help text punctuation

[tomatotomata]

Summary

• tolerate a single final-period difference in metric help text when collectors from different client versions share a registry
• keep all other help-text mismatches strict
• cover both the compatibility case and the strict rejection case

The motivating case is the go_collector change from Sourced from %s to Sourced from %s.. This keeps metric name, type, and labels strict while allowing that known punctuation-only version discrepancy.

Validation

• go test ./prometheus -run 'TestGather(AllowsHelpTextFinalPeriodDifference|RejectsOtherHelpTextDifference)' -count=1
• go vet ./prometheus
• gofmt and git diff --check

This is a draft because I wanted to confirm the compatibility boundary with maintainers before making it ready for review.