FeedResponse<T>.Headers empty for ordered, cross-partition queries

[ccurrens]

Response headers are missing in FeedReponse<T> when the query executed contains an OrderBy statement and is cross-partition.

To Reproduce
Here's a simplified case:

public class Family
{
    [JsonProperty(PropertyName = "id")]
    public string Id { get; set; }

    public string LastName { get; set; }

    [JsonProperty(PropertyName = "_pk")]
    public string PartitionKey => LastName;
}

private Task RunQuery(IQueryable<Family> query)
{
    var iterator = query.ToFeedIterator();
    while (iterator.HasMoreResults)
    {
        var response = await iterator.ReadNextAsync();
        Console.WriteLine(response.Headers.Get("x-ms-documentdb-query-metrics") ?? "MISSING");
    }
}

// Writes "MISSING" to the console
RunQuery(container.GetItemLinqQueryable<Family>().OrderBy(f => f.Id));

// Writes query metrics to the console
RunQuery(container.GetItemLinqQueryable<Family>());

Expected behavior
Headers from the response should be available in the FeedResponse

Actual behavior
There are no headers available in the FeedResponse

Environment summary
SDK Version: 3.17.1 (Also in 3.16, and possibly earlier)
OS Version (e.g. Windows, Linux, MacOSX): All

Additional context
The headers are available by adding a RequestHandler to the client. This is a complicated workaround when the client is used by multiple threads and you are trying to relate requests to one another.

Since the headers are available when running an ordered query in-partition, it seems to be an issue with additionalHeaders not flowing through to the QueryPage result (also here)

Ultimately, though, I think this is indicative of a missing feature in the SDK, where it would be really nice to have the QueryMetrics class (or similar) publicly exposed on responses. Apparently, something similar was available in an earlier SDK version. Our particular use case is structured logging, where we enrich our logs with additional properties, including the index utilization ratio, request counts, and retrieved/output document counts.

[j82w]

@ccurrens please upgrade to the latest 3.18.0 release.