Preserve the type of application builder in chained extension methods

[sliekens]

This is a slight improvement to the usability of extensions in the ServiceDefaults project template. Previously, you couldn't chain WebApplication.CreateBuilder and the ServiceDefaults extensions.

For example, this code won't compile:

var builder = WebApplication.CreateBuilder(args)
    .AddServiceDefaults();

var app = builder.Build();
               // ^^^^^
               // CS1061: 'IHostApplicationBuilder' does not contain a definition for 'Build' and no accessible extension method 'Build' accepting a first argument of type 'IHostApplicationBuilder' could be found

After this change, compilation will succeed.

Microsoft Reviewers: Open in CodeFlow

[eerhardt]

@mitchdenny @DamianEdwards @davidfowl - thoughts here?

If we take this change, we need to update ServiceDefaults everywhere - playground apps, all the templates, etc.

[mitchdenny]

I don't have a problem with this change, but the PR will need to include the fixes to playground apps etc as @eerhardt has identified.

I would defer to folks who have had to deal with the reality of project template changes over time and what issues that can introduce though.

[DamianEdwards]

This seems like something we should consider for 9.0

[mitchdenny]

@sliekens did you want to add the playground app updates here too?

[sliekens]

Yeah I realized after creating the PR that the ServiceDefaults template was not the only place to make changes, though I'm not sure how many other projects would need to be changed.

[mitchdenny]

If you look under /playground you'll see that we generally share a single service defaults project there.

[sliekens]

@mitchdenny I'm noticing other kinds of drift between the different copies of the Extensions.cs, is it okay if I re-align them while I'm making this change?

.e.g. #4096 removed broken Prometheus examples from the templates but they are still present in the playground apps.

[sliekens]

@DamianEdwards thanks for the review, I removed the unused directives and I updated every usage I could find of IHostApplicationBuilder in the playground.