Better metrics per build task, so we can compare build efficiency

[ericoporto]

Description

Right now, total time, which varies with a lot of things, is the only course metric available when looking into an aggregate of tasks (a build), and improves in efficiency on a group of tasks are harder to compare. Ideally, it would be nice to know how much in compute credits fractions a single build consumed, or to have some additional measure of efficiency.

Let's say a PR is made in a repository that improves the CI efficiency, it would be nice if there was some metric that would be helpful in a not task-by-task manner.

Another cool feature would be a projection of how much builds are costing, so we can figure strategies to stay below compute credit limits for the month.

Context

Trying to work with the new FOSS project limits.

There is a nice graph in cirruslabs/cirrus-ci-web#572, not sure if it's possible to have something similar per build. If it's in a PR/branch and showed each build we could see if successive commits are improving or not.

I was working on trying to improve efficiency by running tasks in the same container/machine, dropping some builds that aren't completely necessary in this Cirrus build branch here and noticed that since things were taking the same amount of time to build despite my efforts, I was not completely sure if I was improving things or not (for PR adventuregamestudio/ags#2079).