Kj Tsanaktsidis developed unified multi-observer callback support for metrics in the open-telemetry/opentelemetry-cpp repository, focusing on enhancing observability and data collection efficiency. By introducing Meter::RegisterCallback and Meter::DeregisterCallback, Kj enabled a single callback to observe multiple instruments through a new MultiObserverResult, reducing redundant computations and callback overhead. This C++ API design streamlines metric pipelines, particularly benefiting high-cardinality workloads in production environments. The work demonstrated depth in both software engineering and metrics instrumentation, aligning with 2025 performance goals. No bugs were addressed during this period, as the primary emphasis was on robust feature delivery and scalable API improvements.