Get Total requests in a period of time

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.

SO won't let me comment on Yoory's answer so I have to make a new one...

In Grafana 5.3, they introduced $__range for Prometheus that's easier to use:

sum(rate(http_requests_total[$__range]))

This variable represents the range for the current dashboard. It is calculated by to - from

http://docs.grafana.org/features/datasources/prometheus/

As per increase() documentation, it is not aggregation operator. Thus, it will give wrong answer. (See note.)

You should use sum_over_time() function which aggregates over time interval.

sum_over_time(http_requests_total[24h])

If you have multiple counters, use sum() operator:

sum(sum_over_time(http_requests_total[24h]))

Note: I have 5 datapoints which has values: 847, 870, 836, 802, 836. (updated every minute)

increase(http_requests_total[5m]) returns 2118.75 

sum_over_time(http_requests_total[5m]) returns 4191