Clean up Gitlab::Usage::Metrics::Aggregates::Aggregate class

What does this MR do and why?

There were some mixed responsibilities in lib/gitlab/usage/metrics/aggregates/aggregate.rb file, that would prevent extracting aggregated metrics feature to metrics instrumentation classes framework. This MR cleans up mentioned file as ground work before transfer to instrumentation classes (see follow up at !98206 (merged))

Screenshots or screen recordings

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How to set up and validate locally

Trigger Service Ping from rails console

[1] pry(main)> Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)

MR acceptance checklist

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