The range aggregation `rate()` supports both log ranges and unwrapped
ranges, e.g.

`rate({app="foo"} [$__interval])`
and
`rate({app="foo"} | unwrap bar [$__interval])`

Since `rate()` was split into multiple `count_over_time()` over total
duration, but `count_over_time()` does not support `unwrap`, unwrapped
rate queries caused panics.

This fix changes the splitting of `rate({app="foo"} | unwrap bar [$__interval]`
into multiple `sum_over_time()` over total duration.

Fixes #6344

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

Co-authored-by: Susana Ferreira <susana.ferreira@grafana.com>