Elasticsearch Version

8.16+ (Reindex metrics introduced in 8.16)

Problem Description

Reindex metrics are currently emitted per slice if slicing is enabled, and emitted per whole operation if slicing is disabled. This leads to confusion because the meaning of metric like es.reindex.duration.histogram are different based on whether slicing is used in a reindex operation. It also leads to inaccurate metrics as the metrics for per operation and per slice are mixed together.

Steps to Reproduce

Add slicing (eg. reindex().setSlices(5).source("source")...) in this test will cause the test to fail with 5 metric counts instead of 1.