Fix LongExemplarAssert hasFilteredAttributesSatisfyingExactly to enforce exact attribute matching

[thswlsqls]

Fixes #8517

Description

• LongExemplarAssert.hasFilteredAttributesSatisfyingExactly(AttributeAssertion...) delegated to hasFilteredAttributesSatisfying (the non-exact helper), so it did not reject extra filtered attributes.
• It now delegates to the exact Iterable overload (AssertUtil.assertAttributesExactly), matching DoubleExemplarAssert.hasFilteredAttributesSatisfyingExactly and the same-class Iterable overload.
• Present since 1.21.0; existing longGaugeFailure cases threw for other reasons and never exercised the exactness path.

Testing done

• Added a negative case to MetricAssertionsTest.longGaugeFailure asserting a single filtered attribute on an exemplar that has nine; it must raise AssertionError. Fails before the fix, passes after.
• ./gradlew :sdk:testing:check — passed (16 tests in MetricAssertionsTest).
• No public API change: method signature is unchanged, so no docs/apidiffs update.
• Added a ## Unreleased #### Testing entry in CHANGELOG.md (assertion now stricter; previously lax tests may newly fail).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (c5c9d87) to head (7bcf1d0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8518      +/-   ##
============================================
+ Coverage     91.60%   91.61%   +0.01%     
- Complexity    10312    10313       +1     
============================================
  Files          1013     1013              
  Lines         27274    27274              
  Branches       3202     3202              
============================================
+ Hits          24983    24986       +3     
+ Misses         1563     1561       -2     
+ Partials        728      727       -1     

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[github-actions]

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