Remove mut self reference from SpanExporter::export() method.

[lalitb]

Changes

To make it consistent with LogExporter as done in #2380. This doesn't affect OTLP exporters, as their export are stateless. If exporter need to modify its internal state during export process (e.g. batching etc.), it should rely on the interior mutability.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 18.75000% with 26 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (c072629) to head (ec65383).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/tonic/trace.rs	0.0%	11 Missing ⚠️
opentelemetry-stdout/src/trace/exporter.rs	0.0%	10 Missing ⚠️
opentelemetry-otlp/src/span.rs	0.0%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/http/trace.rs	0.0%	1 Missing ⚠️
opentelemetry-sdk/src/trace/runtime_tests.rs	0.0%	1 Missing ⚠️
opentelemetry-zipkin/src/exporter/mod.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2695     +/-   ##
=======================================
- Coverage   79.2%   79.2%   -0.1%     
=======================================
  Files        123     123             
  Lines      22565   22572      +7     
=======================================
+ Hits       17889   17890      +1     
- Misses      4676    4682      +6     

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[cijothomas]

Nice!