Reexport the built-in MetricsAdapters from the app folder

[Windvis]

This ensures that Embroider doesn't strip the needed built-in adapters out of the build when staticAddonTrees is enabled. We still strip out unused adapters manually.

Closes #316

[Windvis]

I experimented with an @embroider/macros implementation but I couldn't get a version to work that supported stripping out the unused adapters on both a classic and Embroider optimized build, so I went for a more old school approach 😄.

[jherdman]

Many thanks! I'm a little stunned at how easy this scenario was to re-enable. I admit I feel a bit like a goof for not thinking of this sooner! 😞

[Windvis]

@jherdman haha don't be so hard on yourself. It took me a couple of hours experimenting with the @embroider/macros before realizing this setup should work just fine and is way simpler 😄.

Thanks for the merge!

[gwak]

Hello @jherdman, are there plans to release a new version on npm with those changes ?

[jherdman]

Done!