sdk::resource::Resource::Merge should be const

[esigo]

Fixes #1898 (issue)

Changes

changes sdk::resource::Resource::Merge to const.
Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1905 (8d2464c) into main (d557191) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1905      +/-   ##
==========================================
- Coverage   85.72%   85.71%   -0.01%     
==========================================
  Files         171      171              
  Lines        5252     5252              
==========================================
- Hits         4502     4501       -1     
- Misses        750      751       +1     

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/resource/resource.h	100.00% <ø> (ø)
sdk/src/resource/resource.cc	96.30% <100.00%> (ø)
sdk/src/trace/batch_span_processor.cc	90.70% <0.00%> (-0.77%)	⬇️

[lalitb]

LGTM. Thanks for the fix.

[marcalff]

LGTM