test(sdk): reduce span accounting stress test to 10k spans per thread

Addresses review feedback: 40k total spans still exercises the
received+dropped==emitted invariant without adding a 400k-span stress test
to the normal unit suite. The targeted race tests cover the race directly.

Author: Wassbdr

opentelemetry-sdk/src/trace/span_processor.rs

@@ -1524,7 +1524,7 @@ mod tests {
 
         let processor = BatchSpanProcessor::new(exporter, config);
 
-        let total_spans_per_thread = 100_000;
+        let total_spans_per_thread = 10_000;
         let num_threads = 4;
         let total_spans_to_emit = total_spans_per_thread * num_threads;