rack instrumentation: continue trace from distributed trace digest only when distributed trace is present

[anmarchenko]

What does this PR do?
Currently Rack always tries to continue from distributed trace when distributed tracing is enabled, even when there is no distributed trace present.

It causes Rack to always start a new trace, which causes issues with tracing during rails integration tests: controller spans are never appended to a test trace.

Motivation:
A bug reported by Datadog Test Optimization customer: spans from rails instrumentation are not appearing in trace view with a test like that:

  class WelcomeControllerTest < ActionDispatch::IntegrationTest
    test "should get index" do
      get root_url
      assert_response :success
    end
  end

Change log entry
Yes. Fix: rails spans now appear in test trace view when using Datadog Test Optimization with ActionDispatch::IntegrationTest

How to test the change?
Tested using a reproducible case from customer: https://github.com/anmarchenko/test-datadog-ci

Before this parch:

After this patch:

[datadog-datadog-prod-us1]

Datadog Report

Branch report: anmarchenko/fix_rack_distributed_trace_propagation
Commit report: a5be9df
Test service: dd-trace-rb

✅ 0 Failed, 20595 Passed, 1371 Skipped, 3m 12.82s Total Time

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (89e2082) to head (a5be9df).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4398      +/-   ##
==========================================
+ Coverage   97.70%   97.71%   +0.01%     
==========================================
  Files        1361     1361              
  Lines       83246    83246              
  Branches     4226     4227       +1     
==========================================
+ Hits        81336    81347      +11     
+ Misses       1910     1899      -11     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-18 12:41:22

Comparing candidate commit a5be9df in PR branch anmarchenko/fix_rack_distributed_trace_propagation with baseline commit 89e2082 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.