Nit cleanup of method signature for async export

[cijothomas]

Following https://github.com/open-telemetry/opentelemetry-rust/pull/2662/files#r1955258385 comment.

[codecov]

Codecov Report

Attention: Patch coverage is 10.52632% with 85 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (bc931b1) to head (19da2da).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/logs.rs	0.0%	34 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/logs.rs	0.0%	23 Missing ⚠️
opentelemetry-stdout/src/logs/exporter.rs	0.0%	18 Missing ⚠️
opentelemetry-appender-tracing/src/layer.rs	0.0%	5 Missing ⚠️
opentelemetry-otlp/src/logs.rs	0.0%	4 Missing ⚠️
opentelemetry-sdk/src/logs/in_memory_exporter.rs	90.9%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2669   +/-   ##
=====================================
  Coverage   79.1%   79.2%           
=====================================
  Files        122     122           
  Lines      22560   22530   -30     
=====================================
- Hits       17858   17853    -5     
+ Misses      4702    4677   -25     

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[lalitb]

Thank you for cleanup.