code: func WithVars(ctx context.Context, r *http.Request) context.Context {
  	return &muxVarsContext{
  		Context: ctx,
  		vars:    getVarsFromRequest(r),
  	}
  }
docs: // WithVars extracts gorilla/mux vars and makes them available on the returned
  // context. Variables are available at keys with the prefix "vars.". For
  // example, if looking for the variable "name", it can be accessed as
  // "vars.name". Implementations that are accessing values need not know that
  // the underlying context is implemented with gorilla/mux vars.