Alias api types to root

[tensorchen]

See #371

Fixes #371

[lizthegrey]

otel.SetGlobalMeterProvider is really not much more concise than global.SetMeterProvider.

Would support this with the passthrough of Tracer, Meter, and Propagator only, or perhaps some of the key package types.

[jmacd]

See also #295

[lizthegrey]

Approving cautiously. curious what other approvers think.

[jmacd]

This looks OK to me. I would be inclined (if possible) to look at any real code (including examples) to see which can drop a substantial number of import statements as a result of this. I suspect core.KeyValue is going to be a popular reason that packages continue to import core. I suspect MeterProvider and TracerProvider will make that list, too. This is a fine start, and I know that having a non-empty package in this location will help with certain tooling, which want to see code in the root of a package.

[Aneurysm9]

I agree with @jmacd. I don't think it can do any harm and it should help reduce friction with some tooling, so +1.

[MrAlias]

I think this is a first good step. I'm with @jmacd in seeing if this can strategically be progressed.