fix(appender-tracing): don't export synthesized event names as EventName

[miton18]

Fixes #3797
Design discussion issue (if applicable) #3797

Changes

tracing's event macros name an unnamed event concat!("event ", file!(), ":", line!()). The bridge forwarded that name as the record's EventName, so every plain log was exported as an Event whose "type" is a source location that changes with each edit above the call site.

The bridge now sets EventName only when the name was given explicitly (name: "..."). A name is treated as synthesized iff it equals "event " + meta.file() + ":" + meta.line(). The check is exact, uses the Metadata the bridge already holds, and does not allocate. Logger::event_enabled receives the same Option that gets exported, so None for those records.

• layer.rs: is_synthesized_event_name, used in on_event. The existing "target, eventname are not explicitly provided" test now asserts that no EventName is set, instead of asserting the synthesized one.
• lib.rs: the mapping table no longer says every tracing event is an OTel Event.
• CHANGELOG.md: listed as breaking, since anyone filtering on or reading the synthesized name sees it disappear.

Out of scope, noted in the issue: the fixed "log event" name that tracing-log gives records bridged from the log crate.

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)

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Waiting on the author · refreshed 2026-10-17 01:45 UTC

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[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.2%. Comparing base (02b2f15) to head (e2b13c4).

Files with missing lines	Patch %	Lines
opentelemetry-appender-tracing/src/layer.rs	93.7%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3798     +/-   ##
=======================================
- Coverage   84.2%   84.2%   -0.1%     
=======================================
  Files        123     123             
  Lines      30484   30492      +8     
=======================================
+ Hits       25686   25692      +6     
- Misses      4798    4800      +2     

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[miton18]

The one red job, test (stable, ubuntu-24.04-arm), fails in opentelemetry-otlp (exporter::tonic::tests::test_gzip_compression_succeeds_with_feature), which this PR does not touch; the other test jobs were cancelled by fail-fast, and main is green. Could a maintainer re-run it? I don't have the rights.