Opentracing shim noop cases

[pjanotti]

Fixes #2787

Changes

• Since the TelemetrySpan.NoopInstance has an invalid span context the shim should not raise an exception: it is an expected scenario.
• Added integration tests for the shim, interacting with Activity and various sampling configurations to prevent similar regressions in the future.
• Enabled the shim tests against net462.
• Refactored the configuration of listeners in the tests, so different tests could have different configurations.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes

[codecov]

Codecov Report

Merging #4668 (3cd1ea4) into main (358a1cd) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 3cd1ea4 differs from pull request most recent head 774e0b0. Consider uploading reports for the commit 774e0b0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4668      +/-   ##
==========================================
- Coverage   85.06%   85.03%   -0.04%     
==========================================
  Files         314      314              
  Lines       12747    12721      -26     
==========================================
- Hits        10843    10817      -26     
  Misses       1904     1904              

Impacted Files	Coverage Δ
...OpenTelemetry.Shims.OpenTracing/SpanContextShim.cs	100.00% <ø> (ø)
src/OpenTelemetry.Shims.OpenTracing/SpanShim.cs	81.52% <ø> (+0.67%)	⬆️

... and 4 files with indirect coverage changes

[alanwest]

LGTM

[pjanotti]

CI failures seem unrelated to the change:

https:/open-telemetry/opentelemetry-dotnet/actions/runs/5626234900/job/15246635369?pr=4668#step:5:99

https:/open-telemetry/opentelemetry-dotnet/actions/runs/5626234921/job/15246635843?pr=4668#step:4:22