Add a diagnostic source event that fires when a route is matched

[davidfowl]

• Usually more information becomes available about a request once route is matched. This event should allow diagnostic systems to enlighten the typical "begin request" metadata to include more information about the matched route and more importantly the selected endpoint and associated metadata.

cc @lmolkova @SergeyKanzhelev

I'd expect open telemetry to use this to determine what type of request is being routed instead of the specific MVC event:

https://github.com/open-telemetry/opentelemetry-dotnet/blob/3538e05f0eed9df008d30e82a70bef11bd579bb5/src/OpenTelemetry.Collector.AspNetCore/Implementation/HttpInListener.cs#L108-L137

You could also imagine crawling through the endpoint metadata to light up more information about the request (like the gRPC method or controller method that got selected).

[rynowak]

once route is matched

What's a route

[davidfowl]

Man this broke a bunch of tests 😂

[JamesNK]

@davidfowl Is this PR what you refer to here: grpc/grpc-dotnet#341 (comment)

So would the idea be that a diagnostic framework could listen for this event, get the endpoint, then inspect the endpoint metadata to figure out that it is a gRPC/MVC/SignalR request?

[lmolkova]

1. Is it correct that this event will fire for both: WebApi and MVC?
2. How do we get route from HttpContext? httpContext.GetEndpoint()?
3. AppInsights uses https://github.com/aspnet/Mvc/blob/master/src/Microsoft.AspNetCore.Mvc.Core/MvcCoreDiagnosticSourceExtensions.cs events to know about routing today. Should we stop listening to those?

[JamesNK]

1. It will fire for any framework that matches using endpoint routing. Today that means MVC, razor pages, gRPC, SignalR. Likely community frameworks will use it as well. On the endpoint there will be metadata that will let you figure out what the route is to. MVC for example will put the ActionDescriptor in the endpoint metadata. gRPC adds its own metadata, etc.
2. Yes
3. It should still be used for legacy apps. You should be able to listen for both side-by-side. I can't imagine why an ASP.NET Core app would use both old and new routing.

[davidfowl]

@aspnet-hello triage https://dev.azure.com/dnceng/public/_build/results?buildId=244354

[aspnet-hello]

This comment was made automatically. If there is a problem contact aspnetcore-build@microsoft.com.

I've triaged the above build. I've created/commented on the following issue(s)
https://github.com/aspnet/AspNetCore-Internal/issues/2705

[davidfowl]

Hmm preview7 or 8...

[davidfowl]

7