lib/httpserver: option for disabling HTTP keep-alives (#9125)

### Describe Your Changes

Some network configurations may not work optimally with long-lived
connections. \
Address the issue described by #2395 for other components.

### Checklist

The following checks are **mandatory**:

- [x] My change adheres to [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/victoriametrics/contributing/#pull-request-checklist).

---------

Signed-off-by: Aleksandr Frolov <fxrlv@nebius.com>
Co-authored-by: Aliaksandr Valialkin <valyala@gmail.com>
Co-authored-by: Max Kotliar <kotlyar.maksim@gmail.com>
Co-authored-by: Roman Khavronenko <hagen1778@gmail.com>
Co-authored-by: Roman Khavronenko <roman@victoriametrics.com>

lib/httpserver/httpserver.go

@@ -54,6 +54,7 @@ var (
 	flagsAuthKey     = flagutil.NewPassword("flagsAuthKey", "Auth key for /flags endpoint. It must be passed via authKey query arg. It overrides -httpAuth.*")
 	pprofAuthKey     = flagutil.NewPassword("pprofAuthKey", "Auth key for /debug/pprof/* endpoints. It must be passed via authKey query arg. It overrides -httpAuth.*")
 
+	disableKeepAlive            = flag.Bool("http.disableKeepAlive", false, "Whether to disable HTTP keep-alive for incoming connections at -httpListenAddr")
 	disableResponseCompression  = flag.Bool("http.disableResponseCompression", false, "Disable compression of HTTP responses to save CPU resources. By default, compression is enabled to save network bandwidth")
 	maxGracefulShutdownDuration = flag.Duration("http.maxGracefulShutdownDuration", 7*time.Second, `The maximum duration for a graceful shutdown of the HTTP server. A highly loaded server may require increased value for a graceful shutdown`)
 	shutdownDelay               = flag.Duration("http.shutdownDelay", 0, `Optional delay before http server shutdown. During this delay, the server returns non-OK responses from /health page, so load balancers can route new requests to other servers`)
@@ -165,6 +166,7 @@ func serveWithListener(addr string, ln net.Listener, rh RequestHandler, disableB
 
 		ErrorLog: logger.StdErrorLogger(),
 	}
+	s.s.SetKeepAlivesEnabled(!*disableKeepAlive)
 	if *connTimeout > 0 {
 		s.s.ConnContext = func(ctx context.Context, _ net.Conn) context.Context {
 			timeoutSec := connTimeout.Seconds()