WCF: Misc cleanup

[CodeBlanch]

Changes

A bit of misc cleanup:

• Sealed a couple of internal classes.
• Removed the propagator from WcfInstrumentationOptions.
• Instrumentation now resets Activity.Current if it was changed and clears baggage at the end of a request.

TODOs

• CHANGELOG update

[codecov]

Codecov Report

Merging #163 (95b5ee0) into main (91a1269) will decrease coverage by 0.08%.
The diff coverage is 83.33%.

❗ Current head 95b5ee0 differs from pull request most recent head 9f440c4. Consider uploading reports for the commit 9f440c4 to get more accurate results

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   78.78%   78.69%   -0.09%     
==========================================
  Files          85       85              
  Lines        2102     2098       -4     
==========================================
- Hits         1656     1651       -5     
- Misses        446      447       +1     

Impacted Files	Coverage Δ
...strumentation.Wcf/Implementation/ActionMetadata.cs	100.00% <ø> (ø)
...entation.Wcf/TelemetryContractBehaviorAttribute.cs	0.00% <ø> (ø)
...b.Instrumentation.Wcf/WcfInstrumentationOptions.cs	80.00% <ø> (-10.00%)	⬇️
...rumentation.Wcf/TelemetryClientMessageInspector.cs	81.81% <80.00%> (-1.08%)	⬇️
...cf/Implementation/WcfInstrumentationEventSource.cs	26.31% <100.00%> (ø)

[cijothomas]

Would be nice to add code comment about the need to restore Activity.Current