Tracing custom tags not being added to mirrored request

@kyessenov When adding custom tags in tracing, the mirrored request does not receive the same custom tags. Below is my config:

  http:
    name: envoy.tracers.datadog
    config:
      collector_cluster: test-tracing
      service_name: test-tracing

static_resources:
  listeners:
    - name: test_listener
      address:
        socket_address: { address: 0.0.0.0, port_value: 80 }
      filter_chains:
        - filters:
            - name: envoy.filters.network.http_connection_manager
              config:
                tracing:
                  custom_tags: [{"tag": "env","literal": {"value": "${ENV}"}}]
                stat_prefix: ingress_http
                route_config:
                  name: test_route
                  virtual_hosts:
                    - name: test_service
                      domains: ["*"]
                      routes:
                        - match: { prefix: "/" }
                          route:
                            cluster: xxx
                            request_mirror_policies: [{cluster: yyy}]
                http_filters:
                  - name: envoy.filters.http.router