Current management of optical communication systems is conservative, manual-based, and time-consuming. To improve this situation, building an intelligent closed-loop control system is becoming an active topic of the industry. One of the key techniques to achieve such a management system is physical layer impairment telemetry, with the help of which the controller can make proper instructions. However, it is challenging to implement an accurate telemetry module due to the complex mechanisms of various impairments. To overcome that, many studies have been done. In this paper, those recent studies are reviewed, and the design of telemetry is discussed systematically. We analyze metrics for evaluating system performance and mechanisms of various impairments comprehensively, which are the theoretical foundations for designing telemetry modules. We then summarize a unified workflow for designing telemetry modules based on the review of previous works. Its effectiveness is then verified by concrete use cases of our previous studies. Finally, we discuss the challenges of deploying machine-learning-based telemetry techniques in optical communication systems.