[CODE HEALTH] fix clang-tidy cppcoreguidelines-pro-type-const-cast warnings

[Anish701]

Fixes #4477

Changes

Resolves all cppcoreguidelines-pro-type-const-cast warnings by eliminating const_cast usage through proper typing .

• examples/http: Accept const T& in HttpTextMapCarrier constructor, removing the need to const_cast request headers.
• ext/http/client/curl: Change SetCurlPtrOption to accept const void*, and silence const_cast warning for string data.
• ext/test/w3c_tracecontext_http_test_server: Copy headers by value instead of const_casting a reference.
• sdk/common/circular_buffer.h: Make PeekImpl() a const method, eliminating the const_cast<CircularBuffer*>(this) in Peek().

• CHANGELOG.md updated for non-trivial changes - N/A this is a trivial code health change
• Unit tests have been added - N/A
• Changes in public API reviewed - yes mentioned in description

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Anish701 / name: Anish Sinha (4297c36)
• ✅ login: Anish701 / name: Anish701 (d2890d8)

[codecov]

Codecov Report

❌ Patch coverage is 36.84211% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.11%. Comparing base (45f2629) to head (e36cf5b).

Files with missing lines	Patch %	Lines
ext/src/http/client/curl/http_operation_curl.cc	7.70%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4482      +/-   ##
==========================================
- Coverage   83.12%   83.11%   -0.00%     
==========================================
  Files         519      519              
  Lines       20256    20254       -2     
==========================================
- Hits        16835    16832       -3     
- Misses       3421     3422       +1     

Files with missing lines	Coverage Δ
...lude/opentelemetry/metrics/multi_observer_result.h	100.00% <ø> (ø)
...lemetry/ext/http/client/curl/http_operation_curl.h	90.91% <100.00%> (-0.39%)	⬇️
...include/opentelemetry/sdk/common/circular_buffer.h	100.00% <100.00%> (ø)
sdk/src/metrics/multi_observer_result.cc	92.31% <100.00%> (-0.41%)	⬇️
ext/src/http/client/curl/http_operation_curl.cc	60.30% <7.70%> (-0.29%)	⬇️

... and 1 file with indirect coverage changes

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[marcalff]

@Anish701

Thanks for the cleanup patch.

Note: The methods MultiObserverResult::ForInstrument and MultiObserverResult::HasInstrument have their parameters changed from const ObservableInstrument* to ObservableInstrument*. The previous const qualifier was misleading because there was a const_cast on the pointer. Also, all existing callers already pass non-const pointers. This is not a breaking change to the API. All unit tests pass successfully.

I don't think this is the proper fix, and it is a breaking change to the API: what if an existing application invokes opentelemetry with a const pointer today ?

Please avoid changing this area, in favor of #4499 which is a different fix.

[marcalff]

CURL is not changing the blob data given to it.

Making a mutable copy is not necessary, and causes overhead.

[Anish701]

Ok I silenced the lint warnings as you suggested using
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-const-cast)

[marcalff]

Thanks for the patch.

Please revert changes related to metrics instruments, to be fixed separately.

Please revert changes related to CURL blob parameters, and silence the warning instead.

Everything else looks ok.

[Anish701]

Thanks for the patch.

Please revert changes related to metrics instruments, to be fixed separately.

Please revert changes related to CURL blob parameters, and silence the warning instead.

Everything else looks ok.

@marcalff Thanks for reviewing! I reverted the metrics related changes. I also reverted the changes to the CURL blob params and silenced the warnings with // NOLINTNEXTLINE(cppcoreguidelines-pro-type-const-cast).

Please let me know if any other changes are needed. Thanks!