Flexible header matching for HTTP propagator

[marcotc]

While working with OpenTelemetry, I noticed their propagator is flexible when working with Rack HTTP headers.

Our HTTP propagator has always only matched headers that are formatted as HTTP_MY_HEADER_NAME. This means it was not possible to perform simple matches, like my-header-name.

Because it's possible to do both at the same time with almost guarantee safety, this PR does that.

The only conflicting case is if both my-header-name and HTTP_MY_HEADER_NAME are present in the environment. In such case, we prefer to perform the exact match: my-header-name.

I believe this will have no effect on current users.

[codecov-commenter]

Codecov Report

Merging #2504 (c1470eb) into master (89d7ebf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2504      +/-   ##
==========================================
- Coverage   98.32%   98.32%   -0.01%     
==========================================
  Files        1124     1124              
  Lines       60629    60637       +8     
==========================================
+ Hits        59614    59621       +7     
- Misses       1015     1016       +1     

Impacted Files	Coverage Δ
...atadog/tracing/contrib/http/distributed/fetcher.rb	100.00% <100.00%> (ø)
...g/tracing/contrib/http/distributed/fetcher_spec.rb	100.00% <100.00%> (ø)
lib/datadog/core/diagnostics/environment_logger.rb	98.42% <0.00%> (-1.58%)	⬇️
spec/datadog/profiling/native_extension_spec.rb	98.74% <0.00%> (+0.62%)	⬆️

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