The Trade Desk transitioned from a custom-built monitoring system to Prometheus, aiming to handle their extensive online advertising platform that processes 11 million requests per second. During a presentation at GrafanaCon, SRE Patrick O'Brien shared insights into the migration process, highlighting challenges such as migrating complex alerts, the need for improved Prometheus documentation, and addressing cardinality issues due to a high number of hosts. O'Brien emphasized the importance of creating a dedicated team, finding internal advocates to promote the new system, and actively participating in the community for better understanding and support. The transition also involved making metric names generic while embedding metadata to manage the overwhelming metrics data more effectively.