[Package Renames] Expose popularity transfers in search

[loic-sharma]

This exposes the popularity transfer data on the /search/query endpoint. It also exposes the popularity transfer metadata on the /search/diag endpoint. This will later be used to monitor popularity transfer's staleness.

I also introduced a new type, PopularityTransferData to remove the usage of SortedDictionary<string, SortedSet<string>> and remove the need to check case insensitivity for popularity transfers.

Addresses NuGet/NuGetGallery#7903

[loic-sharma]

This property's name was incorrect. See:

NuGet.Services.Metadata/src/NuGet.Services.AzureSearch/AuxiliaryFiles/VerifiedPackagesDataClient.cs

Lines 55 to 80 in 64af0b5

	bool modified;
	var data = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
	AuxiliaryFileMetadata metadata;
	try
	{
	using (var stream = await blobReference.OpenReadAsync(accessCondition))
	{
	ReadStream(stream, id => data.Add(stringCache.Dedupe(id)));
	modified = true;
	metadata = new AuxiliaryFileMetadata(
	lastModified: new DateTimeOffset(blobReference.LastModifiedUtc, TimeSpan.Zero),
	loadDuration: stopwatch.Elapsed,
	fileSize: blobReference.Properties.Length,
	etag: blobReference.ETag);
	}
	}
	catch (StorageException ex) when (ex.RequestInformation.HttpStatusCode == (int)HttpStatusCode.NotModified)
	{
	_logger.LogInformation("The blob {BlobName} has not changed.", blobName);
	modified = false;
	data = null;
	metadata = null;
	}
	stopwatch.Stop();
	_telemetryService.TrackReadLatestVerifiedPackages(data?.Count, modified, stopwatch.Elapsed);

[joelverhagen]

nit: I discovered Array.Empty<T>() recently. Seems nice.

[loic-sharma]

Nice find, I like removing boilerplate :D