Escape text format label values

[Mirochill]

Summary

• escape Prometheus text-format label values for backslashes, quotes, and newlines
• keep label values intact while preventing invalid metrics output
• add regression coverage for a label value with backslashes, quotes, and a newline

Fixes #91

Validation

• Not run locally
• Static validation only: git diff --check, git diff --cached --check, git show --stat --oneline --check HEAD