Requesting metrics SQL should work for metrics that have different parents

[shangyian]

Summary

This PR completes the work around requesting metrics SQL so that it still works if we request two metrics that have different parent nodes. In this scenario, we need to build queries for each set of metrics, grouped by their parent nodes, layer in the metric aggregations, and then join on the shared dimension attributes.

This was previously working in the old SQL build, but we didn't fully complete the work in migrating to the v2 build.

Test Plan

Locally

• [ ] PR has an associated issue: #
• [x] make check passes
• [x] make test shows 100% unit test coverage

Deployment Plan

ASAP

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