feat: Report build completed metrics on Ci::Build finished

What does this MR do and why?

We previously did not have clear observability in place for tracking success metrics around report-type CI builds. This MR adds a metrics counter and export for individual metrics related to completed report builds, grouped by status.

While not directly related, this change comes out of incident gitlab-com/gl-infra/production#6313 (closed) where we should take steps to improve the observability around build completion rates per report type.

Eventually breaking this down by scanner would be ideal but that's a future improvement.

Note this generic counter is subject to the same limitations as #329114 but provides a more generic baseline to iterate against.

feature flag rollout issue: #369500 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. export GITLAB_SIMULATE_SAAS=1 > GDK_ROOT/env.runit per https://docs.gitlab.com/ee/development/ee_features.html#act-as-saas
2. Run pipeline containing artifact reports
3. Visit /-/metrics
4. Note counter values for matching report types

MR acceptance checklist

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