Grpc-dotnet: Example of using gRPC with Polly

[JamesNK]

https://github.com/App-vNext/Polly

Intergrating with a HttpDelegatingHandler probably won't work because trailers are not available. Integration will probably have to happen via interceptor.

[AnthonyGiretti]

@JamesNK I made it work properly, for Unary calls, as you mentionned, but no need to use interceptors

```
Func> retryFunc = (request) =>
{
return Policy.HandleResult(r => r.Headers.GetValues("grpc-status").FirstOrDefault() != "0")
.WaitAndRetryAsync(3, (input) => TimeSpan.FromSeconds(3 + input), (result, timeSpan, retryCount, context) =>
{
var grpcStatus = (StatusCode)int.Parse(result.Result.Headers.GetValues("grpc-status").FirstOrDefault());
Console.WriteLine($"Request failed with {grpcStatus}. Retry.");
});
};

services.AddGrpcClient(o =>
{
o.Address = new Uri("https://localhost:5001");
}).AddPolicyHandler(retryFunc);

[JamesNK]

It will work if no content has been written because grpc-status is in the headers. This is fine for unary methods.

A streaming method that throws an error after it has written a message has the status in trailers, and they will not be available in the delegating handler.

[AnthonyGiretti]

You are right. Thank you.
I only used unary calls.

I will provide more examples soon

[Snede]

@AnthonyGiretti Did you ever add Polly examples?

[AnthonyGiretti]

Hi @Snede Here it is:

https://anthonygiretti.com/2020/03/31/grpc-asp-net-core-3-1-resiliency-with-polly/

If you have any question you can write me / email me anytime

[Snede]

Perfect, this is the first thing that comes up when you search gRPC and Polly on Google, so this will surely help others as well!

[JamesNK]

Anthony's blog is a good example. Will skip adding one to this repo.