Fix .with_headers to support multiple k/v pairs

[joshwlewis]

Changes

Currently, .with_headers doesn't work consistently when passing in multiple k/v pairs. For instance, something like this only adds one of the headers:

HttpExporterBuilder::default()
    .with_headers(HashMap::from([
        ("k1".to_string(), "v1".to_string()),
        ("k2".to_string(), "v2".to_string()),
    ]))

This is quite surprising -- only one header is actually accepted, and it's a random choice. The others are lost. To work around this with the existing code, add one k/v at a time like this:

HttpExporterBuilder::default()
    .with_headers(HashMap::from([("k1".to_string(), "v1".to_string())]))
    .with_headers(HashMap::from([("k2".to_string(), "v2".to_string())]));

I've updated the tests to cover the surprising scenario, and adjusted the logic to support multiple k/v pairs.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: cijothomas / name: Cijo Thomas (d38fe41, c0fd292)
• ✅ login: joshwlewis / name: Josh W Lewis (9e7a36e, 4b1b789, 72f3fda, 2795f2f)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.3%. Comparing base (edab145) to head (2795f2f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2699     +/-   ##
=======================================
- Coverage   79.3%   79.3%   -0.1%     
=======================================
  Files        123     123             
  Lines      22657   22660      +3     
=======================================
+ Hits       17975   17977      +2     
- Misses      4682    4683      +1     

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[cijothomas]

Thanks! Could you add a one-line to the changelog for this bug fix.

[joshwlewis]

@cijothomas done. Not sure what the integration tests failure is about, though. Maybe a fluke and worth retrying?