The PromQL query language uses aggregation operators to combine the values of multiple time series into a single result. The question asks for the "total" number of in-flight requests across all instances. The sum() operator is specifically designed for this purpose; it calculates the sum of the values for all time series that match the provided metric name. In this case, sum(apiservercurrentinflightrequests) will add the value from instance "1" (5) and instance "2" (7), resulting in a single value of 12, which represents the total.

A. min(apiservercurrentinflightrequests) returns the minimum value (5) among the time series, not the total.

C. max(apiservercurrentinflightrequests) returns the maximum value (7) among the time series, not the total.

D. sumovertime() is a range vector function that sums values for each series over a time period, not an aggregation across different series.

1. Prometheus Official Documentation. "Querying Prometheus - Aggregation operators". The documentation explicitly states that sum() calculates the sum over dimensions. This directly supports its use for calculating the total from multiple series. (Available at: https://prometheus.io/docs/prometheus/latest/querying/operators/#aggregation-operators, Section: sum (calculate sum over dimensions))

2. Prometheus Official Documentation. "Querying Prometheus - Functions". This section details sumovertime(), clarifying that it operates on a range vector for a single series to calculate the sum of its values over time, which is different from aggregating across multiple series. (Available at: https://prometheus.io/docs/prometheus/latest/querying/functions/#sumovertime)

3. UC Berkeley, CS 162 Operating Systems and System Programming, Lecture 22. "Monitoring (Prometheus)". The lecture notes describe Prometheus metrics and the use of aggregation functions like sum() to combine data from multiple targets, such as different server instances, into a meaningful total. (Available at: https://cs162.eecs.berkeley.edu/static/lectures/22-Monitoring.pdf, Slide 48, "Example PromQL Queries")