Metrics chart tooltip shows wrong unit label and duplicates entries

## Summary

The metrics chart tooltip has two bugs: (1) all series are labeled `per_second` regardless of the actual aggregation unit (e.g., series that should show `per_minute` still display `per_second`), and (2) every series is duplicated in the tooltip — once with `per_second` and once with `per_minute` — resulting in double the expected legend rows.

## Reproduction

1. Open a metrics chart with multiple series (e.g., HTTP status codes grouped by endpoint).
2. Hover over a data point to open the tooltip.

## Expected behavior

- Each series should display its correct unit label (e.g., `per_second` or `per_minute` depending on the configured aggregation).
- Each series should appear exactly once in the tooltip.

## Observed behavior

- All series are labeled `per_second` regardless of actual unit.
- Each series appears twice — once as `per_second` and once as `per_minute` — with identical values, doubling the annotation list.

Action taken on behalf of David Cramer.

Start by locating the metrics chart tooltip entry point and reproduce the issue with multiple series, such as HTTP status codes grouped by endpoint. Trace how each series' aggregation unit is assigned and how tooltip rows are collected; done means each series appears once with its configured unit label.

Written by the indexing model from the issue text.