[image-builder] Increment metric on build completions

[andrew-farries]

Description

Increment the gitpod_image_builder_builds_done_total counter when the image builder completes a build.

This metric was already configured and registered but never used.

Related Issue(s)

Part of #12960

How to test

• Port forward to the image builder in the preview environment (kubectl port-forward svc/gitpod-image-builder-mk3 9500:9500)
• Hit the metrics endpoint (:9500/metrics).
• Trigger an image build.
• See the gitpod_image_builder_builds_done_total counter increase on completion, with the appropriate label on success or failure.

Release Notes

NONE

Documentation

Werft options:

• /werft with-local-preview
  If enabled this will build install/preview
• /werft with-preview
• /werft with-large-vm
• /werft with-integration-tests=all
  Valid options are all, workspace, webapp, ide

[werft-gitpod-dev-com]

started the job as gitpod-build-af-image-build-completion-metric.2 because the annotations in the pull request description changed
(with .werft/ from main)