Istio EnvoyFilter custom header tag example for adding additional metadata to the span. See https://github.com/istio/istio/issues/22457.

envoyfilter-custom-header-span-x-forwarded-user.yaml

---
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: custom-header-span-x-forwarded-user
  namespace: istio-system
spec:
  configPatches:
    - applyTo: NETWORK_FILTER
      match:
        listener:
          filterChain:
            filter:
              name: envoy.http_connection_manager
      patch:
        operation: MERGE
        value:
          typed_config:
            '@type': >-
              type.googleapis.com/envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager
            tracing:
              custom_tags:
                - request_header:
                    default_value: n/a
                    name: x-forwarded-user
                  tag: x-forwarded-user
...