Fix failing aggregate_spec

What does this MR do and why?

Spec file assumed that there is at least one aggregated metric definition present at appointed directory. Due to migration process (see #373830 (comment 1126596388)) there is non, which made spec failed. This commit uses temp directory to test method implementation until more broad clean up will be delivered in follow up MR as part of #370963 (closed)

Resolves #377661 (closed)

Screenshots or screen recordings

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How to set up and validate locally

In rails console

Gitlab::Usage::Metrics::Aggregates::Aggregate.new(Time.current).monthly_data

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