metrics: fix hot read dashboard panel layout

## Bug Report

The `Statistics - hot read` row in `metrics/grafana/pd.json` has an incorrect layout for the last panels.

On `master`, the last three panels are currently arranged as:

```text
Hot cache read entry numbers x=0 y=74 w=12 h=7
Store read cpu x=0 y=81 w=12 h=7
Selector read events x=12 y=81 w=12 h=7
```

This does not overlap, but `Store read cpu` should be placed next to `Hot cache read entry numbers`, so the layout keeps the normal two-column order:

```text
Hot cache read entry numbers x=0 y=74 w=12 h=7
Store read cpu x=12 y=74 w=12 h=7
Selector read events x=0 y=81 w=12 h=7
```

On `release-8.5`, the problem is more visible because `Store read cpu` and `Selector read events` overlap:

```text
Hot cache read entry numbers x=0 y=74 w=12 h=7
Store read cpu x=0 y=81 w=12 h=7
Selector read events x=0 y=81 w=12 h=7
```

When imported into Grafana, the overlapping panels can be automatically moved, causing the last hot-read panels to appear vertically arranged instead of using the expected two-column layout.

## Expected behavior

Move `Store read cpu` to `x=12, y=74`, next to `Hot cache read entry numbers`, and keep `Selector read events` on the next row.