propagate trace processor errors to users instead of internal logging

[svencowart]

Remove internal logging of errors in span processors and tracer provider in favor of returning errors to callers and providing an error handler callback mechanism for background operations.

Changes

• Add error_handler callback to BatchSpanProcessor for background errors (span drops, export failures during timer-based flushes)
• Return errors from shutdown_with_timeout when spans were dropped
• Remove otel_debug/otel_warn/otel_error calls from span processors
• Simplify export error handling by propagating instead of logging
• Update BatchSpanProcessor builder with with_error_handler() method
• Change dropped_spans_count to Arc<AtomicUsize> for shared ownership
• Remove shutdown error logging from TracerProviderInner

This gives users explicit control over error handling rather than relying on internal SDK logging that may be missed or not actionable.

Fixes #3210

Merge requirement checklist

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

[linux-foundation-easycla[bot]]

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• :white_check_mark: login: svencowart / name: Sven Cowart (47fca8e050d213565448a171138a58dcddfc2b3f)