Define constant for status 499 (#1501)

* Support websocket subprotocl.

* use HTTPHeader()

* Define constant for status code 499

Defined `statusClientClosedRequest` constant in `pkg/filters/proxies/httpproxy/pool.go`
and replaced the hardcoded 499 usage with it. This addresses the TODO comment.

Co-authored-by: LokiWager <32408858+LokiWager@users.noreply.github.com>

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: LokiWager

pkg/filters/proxies/httpproxy/pool.go

@@ -42,6 +42,9 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+// statusClientClosedRequest is the status code for Client Closed Request.
+const statusClientClosedRequest = 499
+
 var httpMethods = map[string]struct{}{
 	http.MethodGet:     {},
 	http.MethodHead:    {},
@@ -458,8 +461,7 @@ func (sp *ServerPool) doHandle(stdctx stdcontext.Context, spCtx *serverPoolConte
 		}
 
 		// NOTE: return 499 if client is Disconnected.
-		// TODO: define a constant for 499
-		return serverPoolError{499, resultClientError}
+		return serverPoolError{statusClientClosedRequest, resultClientError}
 	}
 
 	spCtx.stdResp = resp