fix(sdk-metrics): repair ExponentialHistogram tests

[overbalance]

Summary

Fixes several bugs in the ExponentialHistogramAccumulation test suite. Two test cases were defined inside the body of another it() block, which Mocha silently ignores, making them dead code:

• handles permutations of [1, 1/2, 1/4] with maxSize: 2
• handles ascending sequences

These tests have been moved to the correct describe('record', ...) scope so they actually run. Once exposed, four bugs surfaced:

1. maxFill was reset instead of tracked. Every non-zero bucket reset maxFill to 0 instead of recording the index:

   - maxFill = 0;
   + maxFill = i;

2. maxFill >= maxSize / 2 fails for some parameter combos. After downscaling, positive.length can be less than maxSize (e.g., 2 buckets when maxSize=3). The midpoint check should compare against the actual bucket array length, not the max capacity:

   - assert.ok(maxFill >= maxSize / 2);
   + assert.ok(maxFill >= accumulation.positive.length / 2);

3. Count assertions only check an upper bound. We record exactly maxSize + 1 values (all positive, no NaN), so both totalCount and accumulation.count must be exactly that:

   - assert.ok(maxSize + 1 >= totalCount);
   - assert.ok(maxSize + 1 >= accumulation.count);
   + assert.strictEqual(totalCount, maxSize + 1);
   + assert.strictEqual(accumulation.count, maxSize + 1);

4. Sum assertion only checks one direction. The manual sum and accumulation.sum add the same values in the same order, so they are bit-identical:

   - assert.ok(sum >= accumulation.sum);
   + assert.strictEqual(sum, accumulation.sum);

Test plan

• handles ascending sequences passes across all parameter combinations (maxSize in [3,4,6,9], offset in [-5..5], initScale in [0,4], step in [maxSize..4*maxSize))
• handles permutations of [1, 1/2, 1/4] with maxSize: 2 passes
• All 30 ExponentialHistogram tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (d583a22) to head (fc365dc).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6565   +/-   ##
=======================================
  Coverage   95.75%   95.75%           
=======================================
  Files         371      371           
  Lines       12514    12514           
  Branches     2962     2962           
=======================================
  Hits        11983    11983           
  Misses        531      531           

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[dyladan]

Wow great catch. What were you working on where you noticed that?

[overbalance]

@dyladan On my monster mocha -> vitest PR