Add instrumentation to sort_and_expand_all

What does this MR do and why?

Adds the following isnturmentation:

• gitlab_ci_seed_build_variables_total - variable builds in seed phase
• gitlab_ci_seed_build_variables_duration_seconds - duration historgram
• gitlab_ci_variables_sort_and_expand_total - sort/expand
• gitlab_ci_variables_sort_and_expand_duration_seconds - duration histogram

MR acceptance checklist

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