fix(instr-undici): wrong user agent reported if no user agent were set

[YoannMa]

Which problem is this PR solving?

• undici instrumentation wrongly reports the first header's name as the user-agent if no user-agent header was set (default behavior of undici from what I can tell)

Short description of the changes

request.headers.findIndex(h => h.toLowerCase() === 'user-agent') returns -1 if no user-agent header was present.

I added a check to avoid reporting headers[0] as user-agent

Added a test, not sure if it's useful? Especially as it might break in the future if undici decide to add a default user-agent

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: YoannMa / name: Yoann MALLEMANCHE (0004ea7)
• ✅ login: pichlermarc / name: Marc Pichler (79fdc41)

[david-luna]

@YoannMa

Having no user agent header is a scenario that is likely to happen. Thanks for contributing to OpenTelemetry with this bugfix :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.29%. Comparing base (dfb2dff) to head (79fdc41).
Report is 166 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2282      +/-   ##
==========================================
- Coverage   90.97%   90.29%   -0.68%     
==========================================
  Files         146      147       +1     
  Lines        7492     7247     -245     
  Branches     1502     1505       +3     
==========================================
- Hits         6816     6544     -272     
- Misses        676      703      +27     

Files	Coverage Δ
plugins/node/instrumentation-undici/src/undici.ts	96.29% <100.00%> (ø)

... and 53 files with indirect coverage changes