[TT-13142] make responseWriterWithSize satisfy Flusher() (#39)

* make responseWriterWithSize satisfy Flusher()

go.work.sum

@@ -119,8 +119,11 @@ cloud.google.com/go/websecurityscanner v1.5.0/go.mod h1:Y6xdCPy81yi0SQnDY1xdNTNp
 cloud.google.com/go/workflows v1.10.0/go.mod h1:fZ8LmRmZQWacon9UCX1r/g/DfAXx5VcPALq2CxzdePw=
 github.com/antihax/optional v1.0.0/go.mod h1:uupD/76wgC+ih3iEmQUL+0Ugr19nfwCT1kdvxnR2qWY=
 github.com/census-instrumentation/opencensus-proto v0.4.1/go.mod h1:4T9NM4+4Vw91VeyqjLS6ao50K5bOcLKN6Q42XnYaRYw=
+github.com/cespare/xxhash/v2 v2.2.0 h1:DC2CZ1Ep5Y4k3ZQ899DldepgrayRUGE6BBZ/cd9Cj44=
 github.com/cespare/xxhash/v2 v2.2.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/cncf/udpa/go v0.0.0-20220112060539-c52dc94e7fbe h1:QQ3GSy+MqSHxm/d8nCtnAiZdYFd45cYZPs8vOOIYKfk=
 github.com/cncf/udpa/go v0.0.0-20220112060539-c52dc94e7fbe/go.mod h1:6pvJx4me5XPnfI9Z40ddWsdw2W/uZgQLFXToKeRcDiI=
+github.com/cncf/xds/go v0.0.0-20230607035331-e9ce68804cb4 h1:/inchEIKaYC1Akx+H+gqO04wryn5h75LSazbRlnya1k=
 github.com/cncf/xds/go v0.0.0-20230607035331-e9ce68804cb4/go.mod h1:eXthEFrGJvWHgFFCl3hGmgk+/aYT6PnTQLykKQRLhEs=
 github.com/envoyproxy/go-control-plane v0.11.1-0.20230524094728-9239064ad72f/go.mod h1:sfYdkwUW4BA3PbKjySwjJy+O4Pu0h62rlqCMHNk+K+Q=
 github.com/envoyproxy/go-control-plane v0.11.1/go.mod h1:uhMcXKCQMEJHiAb0w+YGefQLaTEw+YhGluxZkrTmD0g=
@@ -145,4 +148,6 @@ golang.org/x/sys v0.12.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/term v0.9.0/go.mod h1:M6DEAAIenWoTxdKrOltXcmDY3rSplQUkrvaDU5FcQyo=
 golang.org/x/term v0.10.0/go.mod h1:lpqdcUyK/oCiQxvxVrppt5ggO2KCZ5QblwqPnfZ6d5o=
 golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
+google.golang.org/appengine v1.6.7 h1:FZR1q0exgwxzPzp/aF+VccGrSfxfPpkBqjIIEq3ru6c=
 google.golang.org/appengine v1.6.7/go.mod h1:8WjMMxjGQR8xUklV/ARdw2HLXBOI7O7uCIDZVag1xfc=

trace/handler.go

@@ -7,6 +7,11 @@ import (
 	"go.opentelemetry.io/otel/trace"
 )
 
+var (
+	// assert that http.Flusher is implemented by responseWriterWithSize since it is to be used in Tyk gateway.
+	_ http.Flusher = &responseWriterWithSize{}
+)
+
 // responseWriterWithSize is a struct that wraps an http.ResponseWriter and keeps track of the size of the response.
 type responseWriterWithSize struct {
 	http.ResponseWriter
@@ -21,6 +26,10 @@ func (rw *responseWriterWithSize) Write(p []byte) (int, error) {
 	return n, err
 }
 
+func (rw *responseWriterWithSize) Flush() {
+	rw.ResponseWriter.(http.Flusher).Flush()
+}
+
 // NewHTTPHandler wraps the provided http.Handler with one that starts a span
 // and injects the span context into the outbound request headers.
 // You need to initialize the TracerProvider first since it utilizes the underlying