This series fixes a metrics visibility gap in Alternator and adds regression coverage.
Until now, BatchGetItem and BatchWriteItem updated global latency histograms but did not consistently update per-table latency histograms. As a result, table-level latency dashboards could miss batch traffic.
It updates the batch read/write paths to compute request duration once and record it in both global and per-table latency metrics.
Add the missing tests, including a metric-agnostic helper and a dedicated per-table latency test that verifies latency counters increase for item and batch operations.
This change is metrics-only (no API/behavior change for requests) and improves observability consistency between global and per-table views.
Fixes#28721
**We assume the alternator per-table metrics exist, but the batch ones are not updated**
Closesscylladb/scylladb#28732
* github.com:scylladb/scylladb:
test(alternator): add per-table latency coverage for item and batch ops
alternator: track per-table latency for batch get/write operations
035aa90d4b