fix(otel): detach context when single-threaded

[dbolduc]

Part of the work (and fix the implementation of) #12880

We write code like:

{
  auto span = MakeSpan("span");
  OTelScope scope(span);
  return AsyncFoo().then([oc = opentelemetry::context::RuntimeContext::GetCurrent(), span] {
    DetachOTelContext(oc);
    EndSpan(span);
    // We do not want `span` to be active at this point
  }).then([] { AsyncBar(); });
}

A problem occurs when the future returns immediately. Then the code looks more like:

{
  auto span = MakeSpan("span");
  OTelScope scope(span);
  AsyncFoo();
  DetachOTelContext(opentelemetry::context::RuntimeContext::GetCurrent()));
  EndSpan(span);
  // We do not want `span` to be active at this point, but it is.
  AsyncBar();
}

---

To make sure DetachOTelContext() ends the active context, we want to own the token associated with that context.

If the code to set the active span is unfamiliar, look at the implementation of opentelemetry::trace::Scope:
https://github.com/open-telemetry/opentelemetry-cpp/blob/3dfcf93c41bb1d487b3d4d1291791ea21a2a38ce/api/include/opentelemetry/trace/scope.h#L32-L34

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (96762ab) 92.99% compared to head (480b3d5) 92.99%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13154   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files        2137     2137           
  Lines      185804   185828   +24     
=======================================
+ Hits       172791   172816   +25     
+ Misses      13013    13012    -1     

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