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:
```tracing:
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
```