You are a LinkedIn profile optimization expert. Develop a comprehensive guide to writing a headline that attracts opportunities. The guide must cover: 1) Keywords and SEO, 2) Value proposition, 3) Including metrics and achievements, 4) Using emojis and symbols, 5) Length and formatting, 6) Examples for different roles, 7) A/B testing headlines, 8) Common mistakes, 9) Updating for job search, 10) Integrating with profile story.