Update disk-buffering module to follow style guide recommendations

[Copilot]

This PR updates the disk-buffering module to fully comply with the project's style guide recommendations, making minimal but focused changes across 61 Java files.

Key Changes Made

Final Keyword Usage

• Added final to 6 public API classes that should be final per style guide:

  • SpanToDiskExporter, SpanFromDiskExporter
  • LogRecordToDiskExporter, LogRecordFromDiskExporter
  • MetricToDiskExporter, MetricFromDiskExporter

• Removed final from 23+ internal and package-private classes that should not be final according to style guide requirements

Static Imports

Added static imports for commonly used constants as recommended:

• TimeUnit.SECONDS, TimeUnit.HOURS, TimeUnit.MILLISECONDS
• Level.INFO, Level.WARNING, Level.FINER

This improves code readability by eliminating repetitive qualified references like TimeUnit.SECONDS.toMillis(30) in favor of SECONDS.toMillis(30).

Class Organization

Fixed class organization issues to follow the recommended order:

• Static fields (final before non-final)
• Instance fields (final before non-final)
• Constructors
• Methods
• Nested classes

Specifically corrected field ordering in SpanToDiskExporter and constructor placement in nested Builder classes.

Testing

All existing tests (118 total) continue to pass, ensuring no behavioral regressions from these style improvements.

The changes follow the principle of minimal modification while ensuring full compliance with the style guide for better code consistency and maintainability.

Fixes #377.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.