Use autoconfigure SPI for trace-propagators extension.

[anuraaga]

While I'm not exactly sure what @kenfinnigan will do with them, if there's a use case I don't see anything wrong with this since the autoconfigure dep isn't leaked at all.

#3284

[ghost]

Nice. Less "manual" configuration code is always better.

[kenfinnigan]

Thanks @anuraaga, looks great.

The only comment is based on what you asked in my original question, about whether the SPI should be a separate artifact. Ideally, it would be separate, but it's not a showstopper.

[anuraaga]

Thanks @kenfinnigan I think splitting SPI makes sense and is consistent with auto instrumentation too. Will look at it in another PR.

[jkwatson]

This seems fine, especially since it doesn't impact using the propagators directly without autoconfigure around.

[jkwatson]

Nice. Less "manual" configuration code is always better.

As long as we don't lose sight of the fact that there are 100% manual configuration use-cases that we must always support as well.

[kenfinnigan]

Thanks @anuraaga!

[jkwatson]

@anuraaga looks like this didn't include the apidiffs changes introduced by this. I'll add it to my CHANGELOG updates before release.