Fix serialization of array-valued scope and resource attributes in Prometheus exporter

[thswlsqls]

Fixes #5987

Description

• In Otel2PrometheusConverter.convertAttributes(), scope and resource attributes were serialized with raw Object.toString(), so array values became [a, b] (no quotes, no escaping) instead of valid JSON.
• Both paths now use the existing toLabelValue(AttributeType, Object) helper, matching the point-attribute path which already JSON-encodes arrays as ["a","b"]. Scalar output (STRING/BOOLEAN/LONG/DOUBLE) is unchanged because toLabelValue returns toString() for them.
• Completes the incomplete fix from Prometheus label conversion refactored to align with spec #7291 (which aligned only the point-attribute path with the spec). Spec: non-string attribute values are encoded as JSON strings (OTel Prometheus compatibility, common attributes).

Testing done

• Added Otel2PrometheusConverterTest#arrayValuedScopeAttributeSerializedAsJson (scope otel_scope_* label) and an array-valued resource attribute case in resourceAttributesAdditionArgs (resource label). Both fail before the fix, pass after.
• ./gradlew :exporters:prometheus:check — 192 tests pass (Otel2PrometheusConverterTest green; one unrelated flaky PrometheusHttpServerTest.fetch_DuplicateMetrics log-assertion fails intermittently and passes on rerun/in isolation).
• Private-method change only: no public API change, no docs/apidiffs diff.
• Added a ## Unreleased entry in CHANGELOG.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (c3a1c95).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8497   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28995    +2     
  Branches       3599     3599           
=========================================
+ Hits          22839    22841    +2     
  Misses         5311     5311           
  Partials        843      843           

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