Set default service.name if missing

[lalitb]

Fixes # (issue)

Changes

The service.name attribute of Resource is mandatory and SDK should set the default value if not specified. This attribute is necessary for certain backends to identify the service that produced the spans.
as in specs:

• service.name
  MUST be the same for all instances of horizontally scaled services. If the value was not specified, SDKs MUST fallback to unknown_service: concatenated with process.executable.name, e.g. unknown_service:bash. If process.executable.name is not available, the value MUST be set to unknown_service.

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 #616 (9f52cff) into main (07f4a96) will decrease coverage by 0.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #616      +/-   ##
==========================================
- Coverage   94.51%   94.50%   -0.02%     
==========================================
  Files         197      197              
  Lines        9101     9131      +30     
==========================================
+ Hits         8602     8629      +27     
- Misses        499      502       +3     

Impacted Files	Coverage Δ
sdk/src/resource/resource.cc	82.60% <87.50%> (-3.11%)	⬇️
sdk/test/resource/resource_test.cc	92.85% <94.11%> (ø)