fix(sdk-trace): make activeSpanProcessor private

[legalimpurity]

Which problem is this PR solving?

This addresses the issue of making activeSpanProcessor private.
Fixes #4792

Short description of the changes

Makes activeSpanProcessor private in BasicTracerProvider.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

A few tests that were accessing the property have been fixed.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[pichlermarc]

Hi, thanks for working on this, would you mind chaging the target to the next branch and rebasing the changes onto next 🙂

All breaking changes from the SDK 2.0 Milestone usually go onto the next branch 🙂

[pichlermarc]

@legalimpurity would you mind just rebasing your two commits onto the current next branch? then we can proceed with the review 🙂

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.96%. Comparing base (32564ad) to head (2175676).
Report is 4 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #4965   +/-   ##
=======================================
  Coverage   93.96%   93.96%           
=======================================
  Files         310      310           
  Lines        8147     8147           
  Branches     1630     1630           
=======================================
  Hits         7655     7655           
  Misses        492      492           

Files with missing lines	Coverage Δ
...elemetry-sdk-trace-base/src/BasicTracerProvider.ts	95.53% <ø> (ø)