[OpenTracing Bridge] Support traceflags-only propagation without parent span

[ChenX1993]

Resolves #3952

In order to support force sampling by some signal from the inbound request without a parent span, e.g jaeger-debug-id

[pellared]

Resolves #3966

Can you please mention the appropriate issue? 😉

[codecov]

Codecov Report

Merging #3998 (a6bbc31) into main (eb2b89f) will decrease coverage by 0.1%.
The diff coverage is 16.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3998     +/-   ##
=======================================
- Coverage   82.1%   82.0%   -0.1%     
=======================================
  Files        175     175             
  Lines      12977   12988     +11     
=======================================
+ Hits       10655   10657      +2     
- Misses      2102    2110      +8     
- Partials     220     221      +1     

Impacted Files	Coverage Δ
bridge/opentracing/bridge.go	63.9% <0.0%> (ø)
bridge/opentracing/internal/mock.go	63.9% <18.1%> (-3.2%)	⬇️

[ChenX1993]

Resolves #3966

Can you please mention the appropriate issue? 😉

Thanks! Corrected.