Add ddsource to #to_log_format

[TonyCTHsu]

What does this PR do?

Add ddsource to #to_log_format, to stay consistent with log injection data for lograge and semantic_logger

[codecov-commenter]

Codecov Report

Merging #3025 (aaea975) into master (50a2035) will increase coverage by 0.00%.
Report is 9 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3025   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files        1324     1324           
  Lines       74830    74849   +19     
  Branches     3404     3406    +2     
=======================================
+ Hits        73425    73446   +21     
+ Misses       1405     1403    -2     

Files Changed	Coverage Δ
...datadog/tracing/contrib/lograge/instrumentation.rb	100.00% <100.00%> (ø)
...tracing/contrib/semantic_logger/instrumentation.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/correlation.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/correlation_spec.rb	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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