Skip W3C multi-tenant tracestate keys when converting to OpenCensus

[thswlsqls]

Fixes #8808

Description

• SpanConverter.mapTracestate(TraceState) passed every entry to Tracestate.Builder.set, which throws IllegalArgumentException: Invalid key fw529a3039@dt for W3C multi-tenant keys (tenant-id@system-id).
• Once such a key arrived in an incoming tracestate header, Tracer.getCurrentSpan(), spanBuilder(...).startSpan() and TextFormat.extract(...) through the shim all failed.
• The @ form is the only asymmetry between OpenTelemetry ArrayBasedTraceStateBuilder.isKeyValid and OpenCensus Tracestate.validateKey (length, value and 32-entry rules match), so entries whose key contains @ are now skipped. This mirrors the OpenCensus to OpenTelemetry direction, where TraceStateBuilder.put already drops entries it cannot represent.
• The delegate OpenTelemetry span keeps the full tracestate; only the OpenCensus view loses the entry.
• Related: W3C Trace Context key format https://www.w3.org/TR/trace-context/#key, OpenCensus compatibility spec https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/opencensus.md

Testing done

• Added SpanConverterTest#testFromOtelSpanDropsMultiTenantTracestateKeys and OpenTelemetryTextFormatImplTest#testExtractWithW3cDropsMultiTenantTracestateKeys (header tracestate: fw529a3039@dt=fw4,congo=t61rcWkgMzE); both assert only congo=t61rcWkgMzE remains and fail with IllegalArgumentException: Invalid key fw529a3039@dt when the fix is reverted.
• ./gradlew :opencensus-shim:check passed (65 tests).
• No CHANGELOG entry (generated at release time); alpha artifact, no apidiff.

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Pull request dashboard status

Merged · refreshed 2026-09-14 18:06 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.28%. Comparing base (77b1ac4) to head (5b1972a).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8810   +/-   ##
=========================================
  Coverage     91.28%   91.28%           
- Complexity    10528    10530    +2     
=========================================
  Files          1008     1008           
  Lines         28465    28468    +3     
  Branches       3617     3617           
=========================================
+ Hits          25984    25987    +3     
  Misses         1685     1685           
  Partials        796      796           

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