Currently, the custom-metrics-apiserver library only implements the custom and external metrics APIs. So, prometheus-adapter needs to rely on both metrics-server and custom-metrics-apiserver codes to implements all three metrics API. However, metrics-server is not meant to be a library, so interfaces and logic may change at any time which can break the projects consuming metrics-server as a library. 

 This was discussed in sig-instrumentation meeting and it was decided to merge metrics-server API machinery code into the custom-metrics-apiserver project. This effort should be done jointly between metrics-server folks and us, and we have some freedom regarding the implementation. We can either reuse custom-metrics-apiserver and rename it to something like metrics-api-machinery or deprecate custom-metrics-apiserver and replace it with a new metrics-api-machinery project. This should be thought through when migrating metrics-server code and thinking about refactoring the existing custom metrics code.