fix(storage): emit delete metric for the Azure blob backend (#6469)

The S3 (`s3_compatible_storage`) and GCS/opendal backends both increment
`object_storage_requests_total{action="delete_object"}` on delete, but the
Azure blob backend did not, so delete activity was invisible in metrics for
Azure-backed deployments.

Increment the counter (and record the request-duration histogram) in
`AzureBlobStorage::delete`. Since the Azure backend has no batch-delete API,
`bulk_delete` already funnels through `delete` one blob at a time, so this
covers both single and bulk deletes with no other changes.