fix: track the time takes to init loser tree and build last in progress batches

[rluvaton]

Which issue does this PR close?

N/A

Rationale for this change

My latest rewrite to SortPreservingMergeStream forgot to remove comment and to count the initial loser tree init + last in progress emit time

What changes are included in this PR?

• Remove outdated comment
• Count loser tree init time
• Count building last in progress record batches time

Are these changes tested?

No, since it is tricky to test that and any attempt can be flaky by design

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.67%. Comparing base (67947b6) to head (db9ed41).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23698      +/-   ##
==========================================
- Coverage   80.67%   80.67%   -0.01%     
==========================================
  Files        1088     1088              
  Lines      367591   367592       +1     
  Branches   367591   367592       +1     
==========================================
- Hits       296563   296546      -17     
- Misses      53374    53385      +11     
- Partials    17654    17661       +7     

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