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

What you need is the increase() function, that will calculate the difference between the counter values at the start and at the end of the specified time interval. It also correctly handles counter resets during that time period (if any).

increase(http_requests_total[24h])

If you have multiple counters http_requests_total (e.g. from multiple instances) and you need to get the cumulative count of requests, use the sum() operator:

sum(increase(http_requests_total[24h]))

See also my answer to that part of the question about using Grafana's time range selection in queries.

I was missing the time precision of milliseconds which was not being shown in the UI, but it was there due to which I couldn't group with the _time column.

truncateTimeColumn(unit: 1s)

from(bucket: "${bucket}")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "RunningSessions")
  |> filter(fn: (r) => r["_field"] == "totalSessionsCount")
  |> truncateTimeColumn(unit: 1s)
  |> aggregateWindow(every: 5s, fn: last, createEmpty: true)
  |> group(columns: ["_time"])
  |> sum(column: "_value")
  |> group()
  |> yield(name: "Total VU Load")