Add date-based partitioning to parquet file paths

[Copilot]

Adds YYYY-MM-DD date prefix to parquet file key structure for date-based partitioning.

Changes

• pkg/parquet/writer.go: Modified createBackendWriter() to insert date between prefix and metric name using strings.LastIndex() to split the path
• pkg/parquet/writer_test.go: Updated test expectations for new path structure
• README.md: Updated directory structure example

Path Structure

// Before
self.currentFile = fmt.Sprintf("%s/%s.parquet", self.prefix, basename)
// Example: prefix/kube_node_stuff/20240307101250.parquet

// After  
// Splits prefix at last "/" and inserts date between prefix and metric name
lastSlash := strings.LastIndex(self.prefix, "/")
// Example: prefix/2024-03-07/kube_node_stuff/20240307101250.parquet

The implementation correctly places the prefix before the date, inserting the date between the prefix portion and the metric name. This enables date-based S3 lifecycle policies and simplifies retention management while maintaining the prefix structure.

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[xDarksome]

prefix should be before the date

[Copilot]

Fixed in 7ac4619. The path structure now correctly places the prefix before the date: /data/<prefix>/<YYYY-MM-DD>/<metric name>/timestamp.parquet