Add metrics for measure delete progress

[leborchuk]

Here we add prometheus metrics to measure deletion progress.

The main metrics are DeleteProcessTotal && DeleteProcessProcessed. They are operation-specific, reported after completed listing operation.

To measure we instrument each operation by specific prometheus object (from promauto space) and increment metrics on each completed operation. The same for the operation latency

[Copilot]

Pull request overview

Adds Prometheus instrumentation to track progress and performance of long-running delete / garbage-collection operations in pkg/proc, along with documentation and unit tests to validate metric behavior.

Changes:

• Introduces new per-bucket/per-operation delete progress and request metrics (pkg/metrics/delete_metrics.go).
• Instruments delete/untrashify handlers to report batch totals, remaining items, processed/deleted/kept counts, and list/delete latency/size.
• Documents the new metrics in README.md and adds unit tests asserting the emitted values.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents new delete/GC metrics and their labels (bucket, operation, stage).
pkg/proc/delete_handler.go	Emits delete/untrashify progress + latency/size metrics during handler execution.
pkg/proc/delete_handler_test.go	Adds tests to verify metrics are recorded for success/failure/retry paths.
pkg/metrics/delete_metrics.go	Defines new Prometheus gauge/counter/histogram vectors and a small tracker helper.
go.mod	Promotes github.com/prometheus/client_model to a direct dependency for test metric inspection.

Comments suppressed due to low confidence (1)

pkg/proc/delete_handler.go:147

• On a dry-run (Confirm == false), delete_process_remaining is left at len(fileList) even though the handler exits immediately. This can make operational dashboards/alerts think a delete is stuck.

Consider resetting delete_process_remaining before returning.

	if !msg.Confirm { // Do not delete files if no confirmation flag provided
		ylogger.Zero.Info().Str("bucket", bucket).Msg("do not perform actual delete files as no confirmation flag provided")
		return nil
	}

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