Improve gRPC testing

gRPC tests seem to run but code coverage reported is only shows 20% of the code tested:

https://codecov.io/gh/getsentry/sentry-dotnet/tree/e2e00a4c51e4a890eadecdcc9989a2526a4a2a35/src/Sentry.AspNetCore.Grpc

```

alculating coverage result...
Generating report 'D:\a\sentry-dotnet\sentry-dotnet\test\Sentry.AspNetCore.Grpc.Tests\coverage.net5.0.opencover.xml'

+---------------------------+--------+--------+--------+
| Module | Line | Branch | Method |
+---------------------------+--------+--------+--------+
| Sentry.AspNetCore | 31.73% | 13.25% | 35.71% |
+---------------------------+--------+--------+--------+
| Sentry.AspNetCore.Grpc | 20.42% | 10.9% | 19.04% |
+---------------------------+--------+--------+--------+
| Sentry.AspNetCore.Tests | 0% | 0% | 0% |
+---------------------------+--------+--------+--------+
| Sentry | 19.79% | 9.96% | 17.58% |
+---------------------------+--------+--------+--------+
| Sentry.Extensions.Logging | 27.87% | 15.74% | 28.2% |
+---------------------------+--------+--------+--------+
| Sentry.Testing | 28.49% | 13.15% | 16.66% |
+---------------------------+--------+--------+--------+

+---------+--------+--------+--------+
| | Line | Branch | Method |
+---------+--------+--------+--------+
| Total | 16% | 10.58% | 14.67% |
+---------+--------+--------+--------+
| Average | 21.38% | 10.5% | 19.53% |
+---------+--------+--------+--------+
```