* fix(otel): inject experimental.clientTraceMetadata into SSR head (#1494)
When `experimental.clientTraceMetadata` is configured in next.config, read
the active OpenTelemetry context via the registered propagator and emit
`<meta name="..." content="...">` tags in the SSR HTML head for each
allow-listed key. Both App Router and Pages Router are covered (dev and
prod servers). `@opentelemetry/api` is resolved as an optional peer at
runtime, so apps without OTel get a zero-cost no-op.
Mirrors Next.js: packages/next/src/server/lib/trace/utils.ts and
packages/next/src/server/app-render/make-get-server-inserted-html.tsx.
* fix(otel): drop unused getOpenTelemetryTraceData export
knip flagged the helper as unused — it is only consumed internally by
`getClientTraceMetadataHTML`. Keep it module-private.
* fix(otel): normalize clientTraceMetadata allow-list to undefined-only
Pick one convention for the optional allow-list across the App Router
pipeline (was mixing `string[] | null` and `string[] | undefined`).
Drops the redundant `?? null` / `?? undefined` coercions and aligns
every server-side type with `ResolvedNextConfig.clientTraceMetadata`,
which is `string[] | undefined`.
Also adds a resolveNextConfig test asserting that an array passed via
experimental.clientTraceMetadata is preserved verbatim, non-string
entries are filtered, and unset / non-array values resolve to
undefined.
James Anderson