fix(instrumentation-undici): fix several header handling handling bugs#2781

benjaminjkraft wants to merge 2 commits intoopen-telemetry:mainopen-telemetry/opentelemetry-js-contrib:mainfrom

makenotion:benkraft.undici-user-agentmakenotion/opentelemetry-js-contrib:benkraft.undici-user-agentCopy head branch name to clipboard