feat(tracer): add more convenience methods for enterings spans

[bantonsson]

Changes

This adds 3 convenience methods to the Tracer trait that allows users to create, start, and enter a scope with an active Span with parent contexts and based on a SpanBuilder.

Right now, the easiest way to enter a Span is to use the convenience method on the tracer that only takes a name and then add tags to the span afterwards. This is problematic in the sense that samplers and processors that take attributes into account will not see these attributes at creation time.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.9%. Comparing base (df412fe) to head (e0260b2).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3251     +/-   ##
=======================================
+ Coverage   80.8%   80.9%   +0.1%     
=======================================
  Files        129     129             
  Lines      23203   23372    +169     
=======================================
+ Hits       18750   18919    +169     
  Misses      4453    4453             

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[cijothomas]

Looks good. lets add a changelog.md entry before merge.