[@vercel/otel] Fix drained span local export

[kakadiadarpan]

Problem

• Drained spans can still hit automatic local OTLP export if request context is lost before span end.

Solution

• Remember drained span ids at start and skip their local export at end.
• Cap remembered ids at 10k.
• Add SDK-level coverage.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
otel-site	Ready	Preview, Comment, Open in v0	Jun 18, 2026 3:17pm