GraphQL: Refactor `PipelineAnalytics` and `PipelineAnalyticsPeriod` types

In order to put in place the API designed in #444468 (closed), we need to refactor the PipelineAnalytics and PipelineAnalyticsPeriod types:

• Add a total field of type PipelineAnalyticsPeriod to PipelineAnalytics
• Add alpha time filters to PipelineAnalytics type
• Change PipelineAnalyticsPeriod type to have label and a count field

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