perf: batch gateway submits and isolate callbacks

gateway/internal/upstream/manager.go

@@ -23,6 +23,7 @@ const (
 
 type Manager struct {
 	APIBaseURL             string
+	EventAPIBaseURL        string
 	HTTPClient             *http.Client
 	SubmitSegmentPublisher SubmitSegmentPublisher
 
@@ -170,11 +171,15 @@ func (m *Manager) ConnectionCounts() (desired int, connected int) {
 }
 
 func (m *Manager) newConnectionPool(channelID string, connectionID string, config queue.UpstreamConfig) *connectionPool {
+	eventAPIBaseURL := m.EventAPIBaseURL
+	if eventAPIBaseURL == "" {
+		eventAPIBaseURL = m.APIBaseURL
+	}
 	return &connectionPool{
 		channelID:    channelID,
 		connectionID: connectionID,
 		config:       config,
-		apiBaseURL:   m.APIBaseURL,
+		apiBaseURL:   eventAPIBaseURL,
 		httpClient:   m.HTTPClient,
 		reporter: func(ctx context.Context, state ConnectionState) error {
 			return m.post(ctx, "/admin/gateway/connections", state)