Narrow grpc metric collecting decorator to just the grpc services

[baroquebobcat]

Summary

Instead of adding the grpc.service metric decorator to all services declared with the builder, scope it to the grpc service.

When I was looking at the decorator usage a while ago, I noticed that the metric decorator was being added globally in a place that I wouldn't expect it to add a global decorator. This narrows that scope to just the grpc service being added.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.