Add a metrics2 source for the Windows Azure Filesystem driver that was introduced with
AzureFileSystemInstrumentation is the new MetricsSource.
AzureNativeFilesystemStore and NativeAzureFilesystem have been modified to record statistics and some machinery is added for the accumulation of 'rolling average' statistics.
Primary new code appears in org.apache.hadoop.fs.azure.metrics namespace.
Credit for this work goes to the early team: Min Wei, David Lao, Lengning Liu and Alexander Stojanovic as well as multiple people who have taken over this work since then (hope I don't forget anyone): Dexter Bradshaw, Johannes Klein, Ivan Mitic, Michael Rys, Mostafa Elhemali, Brian Swan, [~mikelid], Douma Fang, and Chuan Liu.