fix: Correctly handle auth requests when subrequest logging is enabled

[zacharycmontoya]

Fixes a regression introduced by #231

Scenario

When NGINX is configured in a way to send subrequests, such as Authorization Based on Subrequest Result, we generally do not create a span for it. However, when NGINX is configured with log_subrequest on;, we create a RequestTracing object (and a span) for it, which causes a crashing error when we run the new assertion that there may only be one RequestTracing object present.

Fix

Change the behavior so that when we are deciding what span context to inject into headers and register with the security context we 1) do not assert there's only one span and 2) we use the context from the root span.

Additional notes

In the PR I introduced a regression test to match the previous v1.6.2 nginx-datadog behavior. However, that showed that downstream span would become a child span of the auth subrequest instead of the root nginx span. This PR makes a breaking change so that:

1. The downstream span is a child of the root nginx span (instead of the auth subrequest span)
2. The security context is added to the root span (instead of the auth subrequest span)

[cataphract]

I think it's fine as a fix, although I wonder why we create more than one trace, instead of making the new request just a new span under the current trace

[zacharycmontoya]

I think it's fine as a fix, although I wonder why we create more than one trace, instead of making the new request just a new span under the current trace

We do exactly as you say, we have one trace with two spans. However, I think there's some follow-up we can do to improve how we are handling the two separate spans and their data structures to improve our handling of these subrequest.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.72%. Comparing base (6b51d05) to head (a00f092).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   68.61%   68.72%   +0.10%     
==========================================
  Files          57       57              
  Lines        7322     7322              
  Branches     1035     1035              
==========================================
+ Hits         5024     5032       +8     
+ Misses       1796     1789       -7     
+ Partials      502      501       -1     

Files with missing lines	Coverage Δ
src/datadog_context.cpp	67.33% <100.00%> (+2.01%)	⬆️

... and 1 file with indirect coverage changes

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