Generate a 500-word article outlining a robust retention metric framework for SaaS businesses.  The framework should encompass key metrics beyond simple churn rate, considering factors like customer lifetime value (CLTV), monthly recurring revenue (MRR), expansion revenue, and customer engagement.  Explain the interdependencies between these metrics and how they contribute to a holistic understanding of customer retention.  The article should provide practical examples of how to calculate each metric and interpret the results, along with actionable strategies for improving retention based on the insights gained.  Include a discussion of the limitations of individual metrics and the importance of a balanced approach.  Focus on providing a framework that is easily adaptable to different SaaS business models and scales.  The article should be structured with clear headings and subheadings, and should be written for a business audience with a basic understanding of SaaS metrics.