Assert buildObserver on counter and up-down-counter in shared default meter test

[thswlsqls]

Description

• AbstractDefaultMeterTest called buildObserver() only on the two gauge builders.
• Six public metric builders declare buildObserver(), and both no-op meters (DefaultMeter, ExtendedDefaultMeter) override all six. The eight counter and up-down-counter overrides (4 builders x 2 impls) were never executed by any test.
• Adds buildObserver() with record(value) and record(value, Attributes) to the four existing counter and up-down-counter tests, in the same shape as the gauge tests.
• No production, signature, or public API change. Same pattern as Assert single-arg Logger.isEnabled overload in shared default logger test #8483 and Assert String-key setAttribute overloads in shared default logger test #8567.

Testing done

• ./gradlew :api:all:test --tests DefaultMeterTest — 18 passed. ./gradlew :api:incubator:test --tests ExtendedDefaultMeterTest — 19 passed. ./gradlew :api:testing-internal:check passed.
• Coverage gap, so tests pass both before and after the change.
• Gap confirmed real: with a temporary UnsupportedOperationException in DefaultMeter's long-counter buildObserver(), the old test still passed and the new one failed. Probe reverted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.66%. Comparing base (63ebd94) to head (e2d13f9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8611      +/-   ##
============================================
+ Coverage     91.63%   91.66%   +0.02%     
  Complexity    10326    10326              
============================================
  Files          1013     1013              
  Lines         27295    27295              
  Branches       3205     3205              
============================================
+ Hits          25011    25019       +8     
+ Misses         1558     1550       -8     
  Partials        726      726              

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