[Mvc] Add EventSource tracing to application part discovery

[javiercn]

• Right now this is a common issue that our customer hit regularly.
• Debugging it manually is hard and time consuming.
• Adding event source tracing would allow us to quickly see what's going on in different environments and greatly help customers troubleshoot these types of issues.
• We already do this for other things like dependency injection.

[javiercn]

Sample on DI
https://github.com/aspnet/Extensions/blob/master/src/DependencyInjection/DI/src/DependencyInjectionEventSource.cs

[rynowak]

Do you have some examples of bugs/investigations we could short circuit with this?

[javiercn]

Sample issues this would help troubleshoot
#13850
#10331
#4332

To name a few, but there are more.

With tracing we can answer the following questions:

• What assembly is being considered as the application entry point.
• What related parts were found for that assembly.
• What parts where created from those related assemblies.

[davidfowl]

Just and FYI the reason DI has to use event source is because logging isn’t possible. Why not just log it as debug?

[javiercn]

It’s not possible either. The container is not yet built at the time it happens.

[davidfowl]

@javiercn why not log the results instead of logging the process or building those results?

[javiercn]

@davidfowl Because logging information during the process is what it is useful in troubleshooting the issue. When this issue happens the result is typically an empty application parts list, which gives you no clue as to why you ended up there.

[javiercn]

More issues this will help with
#14359
#14374

[mkArtakMSFT]

Moving this to happen now as this will help with future investigations a lot.