[WIP] Update micrometer-meter-provider module to follow style guide

[Copilot]

Style guide compliance update for micrometer-meter-provider module.

After analyzing the current code against the style guide, I've identified the following areas that need updates:

• Analyze current codebase structure and style guide requirements
• Update public API classes to add final keyword where appropriate
• Update non-public classes to remove final keyword where present (style guide says internal/non-public classes should NOT be final)
• Add static imports for commonly used utility methods where appropriate
• Review and fix field declarations to ensure proper final usage
• Ensure proper class organization (fields, constructors, methods, nested classes)
• Review test classes for package-protected visibility
• Verify all changes with build and tests
• Apply spotless formatting

Key findings:

• Several public API classes need final keyword (MicrometerMeterProvider, ScheduledCallbackRegistrar, etc.)
• Test classes appear to follow package-protected visibility correctly
• Static imports can be added for Objects.requireNonNull and other utility methods
• All formatting appears correct (spotlessCheck passes)

Fixes #399.

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