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 78.79%. Comparing base (824334c) to head (d95bcba).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8518      +/-   ##
============================================
+ Coverage     78.77%   78.79%   +0.02%     
- Complexity     8579     8581       +2     
============================================
  Files          1009     1009              
  Lines         28993    28993              
  Branches       3599     3599              
============================================
+ Hits          22839    22845       +6     
+ Misses         5311     5306       -5     
+ Partials        843      842       -1     

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