Add support for compression in http api

The need has been spotted in issue #3687.
Using "NYTimes/gziphandler", the http api responses can now be compressed if required.
The Go API requires compressed response if possible and handle the compressed response.
We here change only the http api (not the UI for instance).

agent/agent_endpoint.go

@@ -701,6 +701,10 @@ func (s *HTTPServer) AgentMonitor(resp http.ResponseWriter, req *http.Request) (
 
 	// Send header so client can start streaming body
 	resp.WriteHeader(http.StatusOK)
+
+	// 0 byte write is needed before the Flush call so that if we are using
+	// a gzip stream it will go ahead and write out the HTTP response header
+	resp.Write([]byte(""))
 	flusher.Flush()
 
 	// Stream logs until the connection is closed.

agent/http.go

@@ -17,6 +17,7 @@ import (
 	"github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/go-cleanhttp"
 	"github.com/mitchellh/mapstructure"
+	"github.com/NYTimes/gziphandler"
 )
 
 // MethodNotAllowedError should be returned by a handler when the HTTP method is not allowed.
@@ -64,6 +65,7 @@ func registerEndpoint(pattern string, methods []string, fn unboundEndpoint) {
 	if endpoints[pattern] != nil || allowedMethods[pattern] != nil {
 		panic(fmt.Errorf("Pattern %q is already registered", pattern))
 	}
+
 	endpoints[pattern] = fn
 	allowedMethods[pattern] = methods
 }
@@ -111,7 +113,10 @@ func (s *HTTPServer) handler(enableDebug bool) http.Handler {
 			metrics.MeasureSince(append([]string{"consul"}, key...), start)
 			metrics.MeasureSince(key, start)
 		}
-		mux.HandleFunc(pattern, wrapper)
+
+		gzipWrapper, _ := gziphandler.GzipHandlerWithOpts(gziphandler.MinSize(0))
+		gzipHandler := gzipWrapper(http.HandlerFunc(wrapper))
+		mux.Handle(pattern, gzipHandler)
 	}
 
 	mux.HandleFunc("/", s.Index)