feat: add coderd_api_websocket_probes_total metric (#25012)

Relates to CODAGT-115

Adds metric `coderd_api_websocket_probes_total`. Every successful
heartbeat for a given path will increment the metric.

Comparing this with `coderd_api_concurrent_websockets` will give an
indication of how many websocket connections are open but in a 'wedged'
state (when heartbeats stopped versus when we closed the connection).

coderd/workspaceapps/proxy.go

@@ -112,6 +112,7 @@ type ServerOptions struct {
 
 	AgentProvider  AgentProvider
 	StatsCollector *StatsCollector
+	WSWatcher      *httpapi.WSWatcher
 }
 
 // Server serves workspace apps endpoints, including:
@@ -765,11 +766,12 @@ func (s *Server) workspaceAgentPTY(rw http.ResponseWriter, r *http.Request) {
 		})
 		return
 	}
-	go httpapi.HeartbeatClose(ctx, s.Logger, cancel, conn)
 
 	ctx, wsNetConn := WebsocketNetConn(ctx, conn, websocket.MessageBinary)
 	defer wsNetConn.Close() // Also closes conn.
 
+	ctx = s.WSWatcher.Watch(ctx, s.Logger, conn)
+
 	agentConn, release, err := s.AgentProvider.AgentConn(ctx, appToken.AgentID)
 	if err != nil {
 		log.Debug(ctx, "dial workspace agent", slog.Error(err))