Uses the metrics service endpoint to POST the download statistic as JSON
instead of directly updating the 'PackageStatistics' table
Fixed Code Analysis Error. Made GetJObject method a static method
Fixed unit test error
Using HTTPClient instead of WebRequest
Also, changed the call to PostDownloadStatistics to Task.Run from await
so that we achieve Fire and Forget even for the request

Needed to add pragma warnings though to prevent the warning of not using
await, from being treated as an error
Reflecting change to Metrics Service to use the 'DownloadEvent' method on the Service
Merge pull request #2260 from NuGet/daravind/PointToMetricsService

Daravind/point to metrics service