Show expected time on CI builds

[bncer]

HI!

I will continue on Display the time of a running build on the queue page. Specifically, show expected time in form of progress bar.

I need some guidance on source code where to implement.

I did some research and my best guess was to implement db.update_build_status's similar logic (naming a new function update_build_running_time) and create running_time column in build table.

[Kobzol]

Hi, we already know for how long has the build been running for, the only other thing we need to figure out how long we expect it to run. So yeah, we will need to store the duration of a given build in the database (well, we could guesstimate it from the time the PR was merged, but that seems hacky and we don't remember that today anyway). So:

1. Add a duration column to Build and update it when the build finishes (GH should tell us the duration or the start/completion date of the build, so should be easy to compute).
2. Create a new SQL query that will scan the last (ordered by creation time) ~10 successfully completed auto builds, and take the average of their duration. Note that it would be great to analyze this query with EXPLAIN ANALYZE (on some non-empty database, e.g. with 1000 mocked PRs/builds), to see what the DB does. We don't want to sequentially scan over all builds in the DB history, so an index might be needed.
3. Call this query once at the status page to compute the expected duration, pass it to the frontend and use it in the PR table.

Note: this only makes sense for auto builds, not try builds, because those can be running various kinds of CI jobs, and we have no way of knowing which jobs they run from bors.

[bncer]

Thank you!
I will split it in multiple PRs.

2. Note that it would be great to analyze this query with EXPLAIN ANALYZE (on some non-empty database, e.g. with 1000 mocked PRs/builds), to see what the DB does.

Yes, let me check what happens