Observability in the Next.js App Router: Field Notes on instrumentation.ts, onRequestError, and the Trace That Leaked Across Requests

Headline: Next.js's instrumentation.ts file and its onRequestError hook give the App Router real distributed tracing and centralized error reporting, but Vercel Fluid Compute reusing one warm instance across concurrent requests means any request state stored outside OpenTelemetry's own context propagation — a module-level variable, a naive...