[management, client] Add management-controlled client metrics push

Allow enabling/disabling client metrics push from the dashboard via
account settings instead of requiring env vars on every client.

- Add MetricsConfig proto message to NetbirdConfig
- Add MetricsPushEnabled to account Settings (DB-persisted)
- Expose metrics_push_enabled in OpenAPI and dashboard API handler
- Populate MetricsConfig in sync and login responses
- Client dynamically starts/stops push based on management config
- NB_METRICS_PUSH_ENABLED env var overrides management when explicitly set
- Add activity events for metrics push enable/disable

client/internal/engine.go

@@ -997,6 +997,8 @@ func (e *Engine) updateNetbirdConfig(wCfg *mgmProto.NetbirdConfig) error {
 		return fmt.Errorf("handle the flow configuration: %w", err)
 	}
 
+	e.handleMetricsUpdate(wCfg.GetMetrics())
+
 	if err := e.PopulateNetbirdConfig(wCfg, nil); err != nil {
 		log.Warnf("Failed to update DNS server config: %v", err)
 	}
@@ -1066,6 +1068,15 @@ func (e *Engine) handleFlowUpdate(config *mgmProto.FlowConfig) error {
 	return e.flowManager.Update(flowConfig)
 }
 
+func (e *Engine) handleMetricsUpdate(config *mgmProto.MetricsConfig) {
+	if config == nil {
+		log.Debugf("no metrics configuration received from management")
+		return
+	}
+	log.Infof("received metrics configuration from management: enabled=%v", config.GetEnabled())
+	e.clientMetrics.UpdatePushFromMgm(e.ctx, config.GetEnabled())
+}
+
 func toFlowLoggerConfig(config *mgmProto.FlowConfig) (*nftypes.FlowConfig, error) {
 	if config.GetInterval() == nil {
 		return nil, errors.New("flow interval is nil")