fix: Fix bug in distributed tracing when excludeNewrelicHeader is set to true

[nr-ahemsath]

When the agent is configured to exclude the newrelic proprietary distributed tracing header on outbound traces (and rely solely on the W3C trace context headers instead), an exception could occur which would show up in agent logs like this:

NewRelic  ERROR: [pid: 5412, tid: 73] InsertDistributedTraceHeaders() failed
System.InvalidOperationException: Nullable object must have a value.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at NewRelic.Agent.Core.DistributedTracing.DistributedTracePayloadHandler.BuildTraceParent(IInternalTransaction transaction)
at NewRelic.Agent.Core.DistributedTracing.DistributedTracePayloadHandler.GetOutboundHeader(DistributedTraceHeaderType headerType, IInternalTransaction transaction, DateTime timestamp)
at NewRelic.Agent.Core.DistributedTracing.DistributedTracePayloadHandler.InsertDistributedTraceHeaders[T](IInternalTransaction transaction, T carrier, Action`3 setter)

This is because the value of Transaction.Sampled was not being set in this scenario.

This PR fixes that issue and adds integration tests to test both configuration options.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.46%. Comparing base (eea6704) to head (ef98dd9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2457   +/-   ##
=======================================
  Coverage   80.46%   80.46%           
=======================================
  Files         450      450           
  Lines       27942    27945    +3     
  Branches     3023     3023           
=======================================
+ Hits        22483    22486    +3     
  Misses       4686     4686           
  Partials      773      773           

Flag	Coverage Δ
Agent	81.44% <100.00%> (+<0.01%)	⬆️
Profiler	71.93% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...stributedTracing/DistributedTracePayloadHandler.cs	80.66% <100.00%> (+0.21%)	⬆️

[chynesNR]

Thanks for finishing this up!