Add a histogram showing accessed pipelines age

Description

Implement two new Prometheus histograms:

1. Pipeline age histogram
2. Build age histogram

We would make an observation every time a build / pipeline gets loaded from the database.

We would ship that in two iterations, pipelines age histogram first, and then we would do the same for builds. The reason why two iterations may be justified is that by implementing a histogram, we would also immediately get a counter to see how many pipelines we actually load every second. This would allow us to reduce the risk of shipping histogram for builds.