In Prometheus, histograms and summaries expose metrics with _sum and _count suffixes to represent total accumulated values and sample counts, respectively. To compute the average request duration over a given time window (for example, 5 minutes), you divide the rate of increase of _sum by the rate of increase of _count:

text{Average duration} = frac{text{rate(http_request_duration_seconds_sum[5m])}}{text{rate(http_request_duration_seconds_count[5m])}}

Here,

http_request_duration_seconds_sum represents the total accumulated request time, and

http_request_duration_seconds_count represents the number of requests observed.

By dividing these rates, you obtain the average request duration per request over the specified time range.

Extracted and verified from Prometheus documentation -- Querying Histograms and Summaries, PromQL Rate Function, and Metric Naming Conventions sections.