[server] Add SLO for image-build completed successfully

[geropl]

We are currently lacking insight into how often image-builds are not completed successfully. Especially we suspect, that rollouts of server might lead to updates about image-builds are being lost.
To proof that, and to be able to hunt this down, we should introduce a metric + SLO.

• Create metric
• Create SLO

[easyCZ]

Just to clarify, the focus is on measuring from the perspective of the server listening for image build status, not from the perspective of image-builder building the image and completing OK?

If that's the case, then the problem we need to solve here can be expressed with the following metrics:

• Counter of image builds completed as observed by Server
• Counter of image build completed as observed by Image manager

The metric we want to measure is the diff between these.

As a bonus, I'd recommend also adding:

• Counter of image builds started as observed by Server
• Counter of image build started as observed by image manager

These allow us further debug if the request didn't actually reach image builder, or if we just lost the update.