Add byte counts to CAS

Description

Adds byte counts to CAS methods. ByteStream Read, BatchReadBlobs, and GetTree all publish to the cas-downloaded-bytes metric, and ByteStream Write and BatchUpdateBlobs publish to the cas-uploaded-bytes metric.

This uses the Counter construct introduced in !365 (merged). It also fixes a small bug: since the empty string is a valid instance name, the Counter now uses None as the default instance name and checks that the instance name is not None when determining whether to pass the instance name as metadata.

Changes included in this merge request

• Update all CAS methods that read data with metrics to count the bytes read/written using the Counter class introduced in !365 (merged)
• Fix a bug in the Counter class where the empty instance name was not being properly passed as metadata
• Rename Counter._counter to Counter._count