Use BatchStreamProcessor for console writer

[friendlymatthew]

• Fixes combination of export + console breaks tokio sleep #46

This commit replaces SimpleSpanProcessor with BatchSpanProcessor for console span exporting.

Today, the SimpleSpanProcessor exports spans synchronously on the current thread, which can cause deadlocks in async contexts:

BatchStreamProcessor runs on a dedicated thread and avoids this issue (see open-telemetry/opentelemetry-rust#868). It is configured here to process spans immediately (scheduled_delay = 0).

[friendlymatthew]

Snapshot tests fail not because the spans are incorrect, but rather they are out of order from the expected. I'm assuming this is because we process spans by batch.

I don't think the orders matter here, so I'll update the snapshots accordingly.

[davidhewitt]

Hmm, I think the order is quite important (especially for single-threaded code) - it would be presumably quite confusing as a developer if prints got mixed up.

I wonder if this means a custom processor sending data over a channel is a better solution than reusing the batch processor.

[adriangb]

Right it sounds to me like what we need is BackgroundThreadProcessor which wraps another processor.

The SimpleSpanProcessor is by it's name supposed to be as simple as possible but that means it doesn't do basic things like pushing work into a background thread.