Fix a bug in prometheus surfacer's HTTP query handler.
Fix a bug in prometheus surfacer's HTTP query handler.

Using a shared "done" channel may result in panic if an ongoing HTTP handler reads the channel before the handler for which the request has actually finished. See https://github.com/google/cloudprober/issues/222 for more details.

PiperOrigin-RevId: 242071124

cloudprober/cloudprober,cloudprober/cloudprober,google/cloudprober,cloudprober/cloudprober,google/cloudprober