Make Context::HasKey const

[eyjohn]

Fixes #670

Changes

• Update Context::HasKey to be const
• Add const qualifier in tests for HasKey, GetValue to validate constness

• Unit tests have been updated

[codecov]

Codecov Report

Merging #671 (25f056f) into main (9a99b97) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
+ Coverage   94.65%   94.66%   +0.01%     
==========================================
  Files         204      204              
  Lines        9387     9387              
==========================================
+ Hits         8885     8886       +1     
+ Misses        502      501       -1     

Impacted Files	Coverage Δ
api/include/opentelemetry/context/context.h	100.00% <100.00%> (ø)
api/test/context/context_test.cc	98.76% <100.00%> (ø)
sdk/src/logs/batch_log_processor.cc	95.00% <0.00%> (+1.25%)	⬆️