Fix prom_read to use a measurement name as a backend key as prom_write (

#47)

service/http.go

@@ -478,7 +478,7 @@ func (hs *HttpService) HandlerPromRead(w http.ResponseWriter, req *http.Request)
  q := readReq.Queries[0]
  for _, m := range q.Matchers {
  if m.Name == "__name__" {
- metric = m.Name
+ metric = m.Value
  }
  }
  if metric == "" {