Fix worker metrics panic caused by inconsistent Prometheus label cardinality

[pablo1664]

Pull Request

Description

This PR fixes a worker panic when metrics are enabled.

Problem
copy_in_progress_obj_bytes is a GaugeVec defined with 3 labels (flow, user, bucket), but the worker was conditionally building labels and sometimes sending only 2 (for example missing user).
This caused:
panic: inconsistent label cardinality: expected 3 label values but got 2

Fix

• Refactored worker copy metric labeling to always provide all expected label keys.
• Added a shared helper used by both:
  • WorkerInProgressBytesInc()
  • WorkerInProgressBytesDec()
• Empty values are now passed as empty strings instead of omitting labels, preventing cardinality mismatch panics.

Changed file:

• pkg/metrics/service.go

Validation

• Targeted tests executed successfully:
  • go test ./pkg/metrics ./service/worker/copy ./service/worker/handler (exit code 0)