You whole attempt has a couple flaws:

1. If your metric is actually a counter, your query should be sum by (customer) (increase(requests[1d])).
2. In Grafana $__interval stand for time corresponding to one "column" on time scale. Based on your description, you probably intended to use $__range instead.
3. To get only last result of the query, and represented as actual table data, and not independent time series, you need to change options Format to Table, and Type to Instant. Those options can be seen underneath the query in your screenshot.

To hide column Time, you can use Transformation Organize field

I found a solution. Don't know why i did not tried this before. I add the transformation called "Add field from calculation" with mode set to "Reduce now" and calculation "Total".

In theory, switch Grafana query editor to the raw mode and write a query where you use a subquery - outer query will just sum data precalculated by inner query:

SELECT SUM(field_from_main_query) FROM (
  <your current Grafana query with GROUP BY time(10s) hostname>
) GROUP BY time(10s)