128 bits trace id

[TonyCTHsu]

What does this PR do?

Support 128-bit trace id. With this change, our trace is backward compatible with 128-bit trace id, which is used for W3C trace context and b3 propagation.

1. Implement backward compatible distributed tracing propagation with 128-bit trace id
2. Implement backward compatible means for sending spans to agent endpoint
3. Enable 128-bit trace id generation with environment variable DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED
4. Option to log 128-bit trace id with environment variable DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED(Not yet supported)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.56236% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.09%. Comparing base (94721a7) to head (b618b70).
⚠️ Report is 7319 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ddtrace/transport/serializable_trace.rb	85.71%	1 Missing ⚠️
lib/ddtrace/transport/trace_formatter.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2543      +/-   ##
==========================================
+ Coverage   98.08%   98.09%   +0.01%     
==========================================
  Files        1166     1166              
  Lines       63832    64198     +366     
  Branches     2849     2861      +12     
==========================================
+ Hits        62607    62974     +367     
+ Misses       1225     1224       -1     

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[marcotc]

We should also adjust the the span.trace_id on all spans in the TraceFormatter, so the TraceFormatter always makes sure the TraceSegment provided is ready to be serialized without any changes.

This simplifies the serializer's logic.

[TonyCTHsu]

I actually hesitate to commit to this change since TraceFormatter is performing at the trace level attributes which is a constraint of agent's API. I would prefer the current approach of decorating SerailizableSpan than mutating Span from TraceFormatter.