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.