Add `usage.read_units` in fetch/query responses

[jhamon]

Problem

Need to update the generated core and surface usage data properly.

Solution

• Make a few adjustments related to tag names in updated OpenAPISpec
• Update data integration tests to assert read_units are present
• Make adjustments to GRPC response parsing to account for the new data attributes

Type of Change

• [x] New feature (non-breaking change which adds functionality)