Should Exporter force async?

[cijothomas]

Exporter traits are now written as async. This works great if the exporter actually needs to use async. But when exporter is not needing async (like when exporting to kernel tracing systems like Windows ETW, Linux user_events), the async is causing unnecessary overhead, as shown in the benchmarks.

This issue is to track addressing this. One way could be offer multiple methods. Another would be model etw/user-events exporter as a processor itself,