chore: Unify SpanExporter with LogExporter and PushMetricExporter

[scottgerring]

Fixes #2262

Changes

Changes SpanExporter trait methods from &mut self to &self:

• shutdown(&self)
• shutdown_with_timeout(&self, timeout)
• force_flush(&self)

Also fixes shutdown() default timeout from 5 nanoseconds to 5 seconds to make it consistent with the others (assume this was a typo!).

Refactors TonicTracesClient to wrap inner in Mutex<Option<...>> for interior mutability, aligning its structure
and export pattern with TonicLogsClient and TonicMetricsClient.

This is breaking change - Implementers of SpanExporter must update method signatures. Those already using interior mutability are good.

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.7%. Comparing base (65969fd) to head (9475348).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/tonic/trace.rs	0.0%	26 Missing ⚠️
opentelemetry-otlp/src/exporter/http/trace.rs	0.0%	1 Missing ⚠️
opentelemetry-sdk/src/trace/export.rs	75.0%	1 Missing ⚠️
opentelemetry-sdk/src/trace/span_processor.rs	50.0%	1 Missing ⚠️
opentelemetry-stdout/src/trace/exporter.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3281     +/-   ##
=======================================
- Coverage   80.7%   80.7%   -0.1%     
=======================================
  Files        129     129             
  Lines      23210   23217      +7     
=======================================
  Hits       18748   18748             
- Misses      4462    4469      +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cijothomas]

Nice cleanups!