Switch to async exporters

[jtescher]

This change updates the `SpanExporter interface to export asynchronously. The primary API change is:

- fn export(&self, batch: Vec<Arc<trace::SpanData>>) -> trace::ExportResult;
+ async fn export(&self, batch: &[Arc<trace::SpanData>]) -> trace::ExportResult;

Implementation notes:

• Switch from Vec<_> to &[_] in exporters to avoid additional allocation when batch exporting.
• Currently uses async_trait, could change method signature to avoid the dependency if preferable.
• BatchSpanProcessorWorker's Future impl now awaits sub-futures so replaced with async block for greater clarity.
• The thrift and grpcio crates are not currently async, so some work still necessary to make jaeger and otlp exporters async (they currently call blocking code in a future, which is not great)

[TommyCpp]

Nice job! I think grpcio generally support async(I could be wrong since I didn't use the async part before). But didn't find thrift async crate. May need to figure out how to make that work.

[jtescher]

@TommyCpp yeah may have to just use the thrift crate to write to a buffer synchronously and then send async

[TommyCpp]

LGTM 👍

[jtescher]

Ok going to see if making the jaeger exporter async is a quick change, if not can do in a follow up PR

[frigus02]

Looks really nice 👍

[jtescher]

@frigus02 good catch, still toying with the jaeger exporter but will fix those as well 🙏

[jtescher]

Ok switched jaeger collector client to use the isahc http client as it is executor agnostic, and manually maintain a buffer for the UDP agent and flush async. Could consider making the zipkin exporter executor agnostic in a future PR, but at least all the exporters are now non-blocking 🎉 .

I also removed the max packet size option for the agent exporter as I don't think you can actually configure the packet size in any of the implementations.

[frigus02]

I'm not familiar with the Jaeger exporter, but as far as I understand these changes look nice.

Is it worth documenting, that users might want to enable the tokio or async-std feature when they're using the Jaeger agent?