make clients of sse service observable

dist/sse.d.ts

@@ -8,8 +8,17 @@ export type SseEvent = {
     data?: unknown;
     message?: string;
 };
+export type SseClientChangeEvent = {
+    type: "add" | "remove";
+    clientId: string;
+    clientCount: number;
+};
+export type SseClientChangeCallback = (event: SseClientChangeEvent) => void;
 export declare class SseService {
     private clients;
+    private clientChangeCallbacks;
+    onClientChange(callback: SseClientChangeCallback): () => void;
+    private emitClientChange;
     addClient(client: SseClient): void;
     removeClient(clientId: string): void;
     notifyClients(event: SseEvent): void;