Generate a full, in-depth guide for SaaS businesses on implementing a Product-Led Growth (PLG) strategy. The guide must be written in a clear, strategic tone. It must include detailed sections on: 1. The Core Principles of PLG vs. Sales-Led Growth. 2. Designing a 'Freemium' or 'Trial' Experience that showcases value instantly. 3. Identifying Product-Qualified Leads (PQLs) using analytics. 4. The evolving role of the sales team in a PLG model. 5. Key PLG metrics to track, such as activation rate and expansion revenue. Please include a disclaimer that PLG is a fundamental business model shift.