Improve gRPC testing

[bruno-garcia]

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% |
+---------+--------+--------+--------+

[kanadaj]

@Mitch528 xP