[bengl]

The undici instrumentation adds traceparent to every outgoing undici and fetch request, and tracestate and baggage when the span has them. It added each one with request.addHeader(), which validates the pair before appending it to undici's flat array of header names and values. traceparent now goes onto that array directly.

Why only traceparent. microtel builds the traceparent value from IDs and flags it generated or already validated, so addHeader() has nothing to catch there, and every request carries one. tracestate and baggage carry values that came from elsewhere, so they keep going through addHeader(). A test checks which names reach it.

Why it's safe. For a lowercase name that isn't one of undici's special headers, such as host or content-length, addHeader() validates the value and pushes the pair as given. undici's HTTP/1 writer and HTTP/2 header builder both read the array in pairs. When the headers aren't an even-length array, the append falls back to addHeader(), so an undici that stores headers differently can't break requests.

Performance. Per appended header, on Node 26.12.1, over 7 interleaved runs:

	Median	Samples (ns)
addHeader()	76.7 ns	75.2, 75.8, 76.5, 76.7, 77.1, 77.4, 77.6
Direct push	16.5 ns	16.0, 16.1, 16.2, 16.5, 16.6, 17.1, 18.3

That's about 60 ns per request, around 0.1% of a traced fetch, which is below this machine's noise in the fetch benchmark.

Tests.

• A fetch carries the full trace context, with each header appended rather than overriding a caller's.
• A fixture spying on addHeader() sees tracestate and baggage but not traceparent.
• Fixtures whose request headers are an object or an odd-length array still send a valid traceparent, through addHeader().