policy.xml

<policies>
    <inbound>
        <base />
        <authentication-managed-identity resource="https://cognitiveservices.azure.com" output-token-variable-name="managed-id-access-token" ignore-error="false" /> 
        <set-header name="Authorization" exists-action="override">  
            <value>@("Bearer " + (string)context.Variables["managed-id-access-token"])</value>  
        </set-header>
        <set-backend-service backend-id="{backend-id}" />
        <azure-openai-emit-token-metric namespace="openai">
            <dimension name="Subscription ID" value="@(context.Subscription.Id)" />
            <dimension name="Client IP" value="@(context.Request.IpAddress)" />
            <dimension name="API ID" value="@(context.Api.Id)" />
            <dimension name="User ID" value="@(context.Request.Headers.GetValueOrDefault("x-user-id", "N/A"))" />
        </azure-openai-emit-token-metric>
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <base />
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>