Stats endpoint API should shut down cleanly

[fearful-symmetry]

cc @cmacknz
In cases where agent is shut down cleanly (for example, with a sigint), we get this message:

{"log.level":"info","@timestamp":"2023-03-08T10:47:12.737-0800","log.logger":"api","log.origin":{"file.name":"api/server.go","file.line":72},"message":"Stats endpoint (127.0.0.1:6791) finished: accept tcp 127.0.0.1:6791: use of closed network connection","log":{"source":"elastic-agent"},"ecs.version":"1.6.0"}

This is a benign error, but it's almost always the last message we get when agent shuts down, which means users might get confused and assume it's somehow related to the shutdown. We should either clarify the error message to make it sound less threatening, or change the (s *Server) Stop() method so we don't print an error during "normal" shutdown.

[cmacknz]

elastic-agent-libs/api/server.go

Lines 66 to 79 in 3fd7788

	// Start starts the HTTP server and accepting new connection.
	func (s *Server) Start() {
	s.log.Info("Starting stats endpoint")
	go func(l net.Listener) {
	s.log.Infof("Metrics endpoint listening on: %s (configured: %s)", l.Addr().String(), s.config.Host)
	err := http.Serve(l, s.mux)
	s.log.Infof("Stats endpoint (%s) finished: %v", l.Addr().String(), err)
	}(s.l)
	}
	// Stop stops the API server and free any resource associated with the process like unix sockets.
	func (s *Server) Stop() error {
	return s.l.Close()
	}

Seems be because we pass a net.Listener to the default http.Server, instead of creating a server and calling close on the server, which would let us check for ErrServerClosed and not log it.

// Serve accepts incoming HTTP connections on the listener l,
// creating a new service goroutine for each. The service goroutines
// read requests and then call handler to reply to them.
//
// The handler is typically nil, in which case the DefaultServeMux is used.
//
// HTTP/2 support is only enabled if the Listener returns *tls.Conn
// connections and they were configured with "h2" in the TLS
// Config.NextProtos.
//
// Serve always returns a non-nil error.
func Serve(l net.Listener, handler Handler) error {
	srv := &Server{Handler: handler}
	return srv.Serve(l)
}

[michel-laterman]

fyi i added #168 to expose shutdown to the caller