I tracked the problem down to the fact that the http.ResponseWriter is
being wrapped by github.com/opentracing-contrib/go-stdlib, causing the
Flusher() method to be masked. As a result, the ResponseWriter does not
satisfy the http.Flusher interface.
I have tested building with a newer version of
opentracing-contrib/go-stdlib (currently pending review by the package
maintainer) and it resolves the issue.
Once the updated golang-github-opentracing-contrib-go-stdlib package is
approved and uploaded, we can build a new prometheus package against it.
