Add flag to control reordering metrics for display

`DisplayableExecutionPlan::indent` always reorder metrics. However, ones might expect the order of the metrics conform to the registration order.

I propose to add a flag to control the reordering behaviour. For example:

```
DisplayableExecutionPlan::new(&plan).set_reorder(false).indent(false)
```

Start by locating DisplayableExecutionPlan::new and indent, then inspect nearby tests or callers to understand the current metric reordering. Add coverage showing that the default behavior remains unchanged and that disabling reordering preserves registration order.

Written by the indexing model from the issue text.