fix: [iceberg] reduce granularity of metrics updates in IcebergFileStream

[mbutrovich]

Which issue does this PR close?

N/A.

Rationale for this change

Profiling of Iceberg table scans revealed significant overhead from timer calls (clock_gettime) in the IcebergFileStream state machine.

What changes are included in this PR?

Move baseline.record_poll() from inside the state machine (where it wrapped every batch poll) to the top-level poll_next() method, matching the DataFusion FileStream pattern:

• Remove nested record_poll() in the Reading state
• Add record_poll() call at the top-level poll_next()

How are these changes tested?

Existing tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.58%. Comparing base (f09f8af) to head (ef9af62).
⚠️ Report is 828 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3050      +/-   ##
============================================
+ Coverage     56.12%   59.58%   +3.45%     
- Complexity      976     1368     +392     
============================================
  Files           119      167      +48     
  Lines         11743    15497    +3754     
  Branches       2251     2573     +322     
============================================
+ Hits           6591     9234    +2643     
- Misses         4012     4966     +954     
- Partials       1140     1297     +157     

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[hsiang-c]

LGTM