A few general comments, it looks like this is breaking at least one test in the asan-pr build. Also, looking at CLang tidy there are some other problems with copy construction there.

That said, as we discussed I'm not sure adding a new function is the way to best fix this. Perhaps this is a good topic to raise in the weekly C++ meeting tomorrow but I would think the existing functions should behave correctly.

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I deleted the previous "quick fix" and added a new class CustomHeaders in GCS.
You can now add multiple custom headers to your request.

example:

    auto response = client.ListObjects(
        "my_bucket",
        gcs::CustomHeaders({{"name1", "value1"}, {"name2", "value2"}})
    );

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.87%. Comparing base (3498e58) to head (f765e10).
Report is 4 commits behind head on main.

@@           Coverage Diff           @@
##             main   #15053   +/-   ##
=======================================
  Coverage   92.86%   92.87%           
=======================================
  Files        2356     2356           
  Lines      210713   210741   +28     
=======================================
+ Hits       195675   195718   +43     
+ Misses      15038    15023   -15     

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@eliekozah can you convert this to a draft pr if you are still working on it. Will help avoid people reviewing it.