[WIP] Update aws-xray module to follow style guide

[Copilot]

Analyzing aws-xray module to identify areas that need updating to follow the style guide. Based on initial examination:

Analysis Plan:

• Examine repository structure and style guide requirements
• Run current build and tests to ensure baseline functionality
• Check current code formatting (spotlessCheck passes)
• Review key classes to identify style guide violations

Key Issues Identified:

• Static imports: Some files may benefit from recommended static imports for AttributeKey, Collections, etc.
• final keyword usage: Some public classes and fields may need final declarations per style guide
• Class organization: Check field/method ordering and private constructor placement
• Import optimization: Remove unused imports and organize properly

Implementation Plan:

• Examine all Java files systematically for static import opportunities
• Add final keyword to public classes where appropriate
• Add final keyword to fields where possible
• Organize class members according to style guide (static fields, instance fields, constructors, methods, nested classes)
• Run spotlessApply and validate tests after each change
• Verify minimal changes approach

Changes Made:

• Added static imports for AttributeKey.stringKey, AttributeKey.longKey in multiple files
• Added static imports for Collections.unmodifiableMap, Collections.emptyMap, Collections.unmodifiableList
• Added static imports for TimeUnit.SECONDS, TimeUnit.NANOSECONDS
• Added static imports for Collectors.toList, Collectors.toSet, Collectors.toMap
• Made public classes final: AwsMetricAttributesSpanExporter, AwsXrayRemoteSamplerProvider, AttributePropagatingSpanProcessorBuilder, AwsMetricAttributesSpanExporterBuilder
• All tests continue to pass

Fixes #236.

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