Need Unit Tests for gRPC server instrumentation

[NathanielRN]

In PR open-telemetry/opentelemetry-python#1308 the Spans created as part of instrumenting gRPC were corrected to use the error codes as recently updated in the OTel Specifications instead of the gRPC error codes.

This didn't break any unit tests which is not ideal, we should add unit tests so that changes like these are flagged in the tests and require a test change.