support gRPC protocol in CollectorExporter

Author: huandu

.gitignore

@@ -1,3 +1,4 @@
 .dart_tool
 .packages
 pubspec.lock
+opentelemetry-proto

lib/sdk.dart

@@ -8,7 +8,8 @@ export 'src/sdk/time_providers/datetime_time_provider.dart'
     show DateTimeTimeProvider;
 export 'src/sdk/time_providers/time_provider.dart' show TimeProvider;
 export 'src/sdk/trace/exporters/span_exporter.dart' show SpanExporter;
-export 'src/sdk/trace/exporters/collector_exporter.dart' show CollectorExporter;
+export 'src/sdk/trace/exporters/collector_exporter.dart'
+    show CollectorExporter, CollectorExporterProtocol;
 export 'src/sdk/trace/exporters/console_exporter.dart' show ConsoleExporter;
 export 'src/sdk/trace/id_generator.dart' show IdGenerator;
 export 'src/sdk/trace/sampling/always_off_sampler.dart' show AlwaysOffSampler;