Add immediate span publishing

[ChiragAgg5k]

Summary

• Add Span::publish() for immediate storage-free publishing to configured exporters.
• Share completion/export logic between publish() and finish() so timestamps, level defaults, samplers, and exporter exception handling stay consistent.
• Document immediate publishing and cover storage isolation, errors, samplers, levels, and validation in tests.

Testing

• composer test
• composer format
• composer analyze

[greptile-apps]

Greptile Summary

This PR adds Span::publish() for fire-and-forget event export without storage interaction, and refactors finish() into two private helpers — complete() for timestamp/level computation and exportSpan() for exporter dispatch — keeping the two call paths consistent. The implementation is clean, attribute validation silently skips invalid keys and values (consistent with the "never break the application" philosophy), and the ten new tests cover all specified behaviors.

Confidence Score: 5/5

Safe to merge — no correctness, security, or behavioral regressions found.

No P0 or P1 issues identified. The refactoring correctly preserves existing finish() semantics, publish() is well-scoped and thoroughly tested, and attribute validation is intentionally lenient rather than throwing.

No files require special attention.

Important Files Changed

Filename	Overview
src/Span/Span.php	Adds publish() static method for storage-free immediate export; refactors finish() by extracting complete() and exportSpan() helpers with no behavioral regressions.
tests/SpanTest.php	Adds 10 focused tests for publish() covering exporters, storage isolation, error handling, level defaults/overrides, sampler filtering, and invalid attribute key/value skipping.
README.md	Adds documentation for Span::publish() with usage examples and adds the method signature to the static API reference table.

_{Reviews (2): Last reviewed commit: "Add immediate span publishing" | Re-trigger Greptile}