This is the thing that really frightens me, since it always starts off so innocuous with web APIs.  It looks good today; then tomorrow needs something different.

I would really feel so much better if we baked in versioning by request header right from the start.  It wouldn't really take too much more work to do, but it'd buy us security as folks start to build tools.  Barring having a single repository for all source code that will use the client library to talk to Prometheus, we will have to be stupendously diligent in supporting legacy clients.