Fix spelling typos in API and metrics test code

[thswlsqls]

No issue: trivial spelling fix, an issue is not required.

Description

• Rename the four Gauage test methods in AbstractDefaultMeterTest to Gauge, matching the LongGauge/DoubleGauge types they exercise.
• Fix the scope name literal "advanced-instrumetnation" to "advanced-instrumentation" in the same class; the no-op MeterProvider discards the name.
• Fix side affect to side effect in an AbstractOpenTelemetryTest comment and // Gauage to // Gauge in a SdkMeterTest comment.
• Spelling only: no behavior, signature, public API, or asserted output changes. api/testing-internal is not published, so there is no API diff.

Testing done

• Trivial typo, test-exempt: no test added.
• ./gradlew :api:testing-internal:check :api:all:test :api:incubator:test passed; test-results XML shows the renamed methods ran under their new names in both subclasses.
• ./gradlew :sdk:metrics:test --tests "*SdkMeterTest" :sdk:metrics:spotlessCheck passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.62%. Comparing base (dd811c4) to head (b774c64).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8689   +/-   ##
=========================================
  Coverage     91.62%   91.62%           
  Complexity    10327    10327           
=========================================
  Files          1003     1003           
  Lines         27138    27138           
  Branches       3187     3187           
=========================================
  Hits          24866    24866           
  Misses         1566     1566           
  Partials        706      706           

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