REST Api for Custom metric API server not in Go

I want to implement custom metric API server to use with HPA. we have proprietary monitoring system that have client library in C# but all the implementation i saw online are in Go.

Is there a definition of REST API (request and response) that the HPA call? or this can be only in Go (for some reason)?

I can use this to create a proxy server between Go and my C# server but i rather go straight to my server from HPA