Describe OTLP exporter usage

[pellared]

Why

Currently, trying to use the OTLP exporter usually leads to problems.

Related issue #325

Related PR #343

What

Describe the current state of the OTLP exporter support. Proper support of even deperecation of the gRPC protocol can be done later.

Remarks

For .NET Framework, it is probably more complicated. When I added

<PackageReference Include="Grpc.Net.Client" Version="2.32.0" />

to the sample app I got:

Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory 
is corrupt.
   at Grpc.Core.Internal.CompletionQueueSafeHandle.ReleaseHandle()
   at System.Runtime.InteropServices.SafeHandle.InternalDispose()
   at System.Runtime.InteropServices.SafeHandle.Dispose(Boolean disposing)
   at System.Runtime.InteropServices.SafeHandle.Dispose()
   at Grpc.Core.Internal.AsyncCall`2.UnaryCall(TRequest msg)
   at Grpc.Core.DefaultCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request)    
   at Grpc.Core.Interceptors.InterceptingCallInvoker.<BlockingUnaryCall>b__3_0[TRequest,TResponse](TRequest req, ClientInterceptorContext`2 ctx) 
   at Grpc.Core.ClientBase.ClientBaseConfiguration.ClientBaseConfigurationInterceptor.BlockingUnaryCall[TRequest,TResponse](TRequest request, ClientInterceptorContext`2 context, BlockingUnaryCallContinuation`2 continuation)
   at Grpc.Core.Interceptors.InterceptingCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request)
   at Opentelemetry.Proto.Collector.Trace.V1.TraceService.TraceServiceClient.Export(ExportTraceServiceRequest request, CallOptions options)      
   at Opentelemetry.Proto.Collector.Trace.V1.TraceService.TraceServiceClient.Export(ExportTraceServiceRequest request, Metadata headers, Nullable`1 deadline, CancellationToken cancellationToken)
   at OpenTelemetry.Exporter.OpenTelemetryProtocol.Implementation.ExportClient.OtlpGrpcTraceExportClient.SendExportRequest(ExportTraceServiceRequest request, CancellationToken cancellationToken)
   at OpenTelemetry.Exporter.OtlpTraceExporter.Export(Batch`1& activityBatch)
   at OpenTelemetry.BatchExportProcessor`1.ExporterProc()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)  
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()
Segmentation fault

[pjanotti]

LGTM - just need to be careful with the changes in #351