COALESCE selected expressions to capture all values

[shangyian]

Summary

For metrics SQL generation, we should COALESCE the selected expressions after doing a FULL OUTER JOIN on the shared dimensions of the metrics. This is because the intention of the FULL JOIN is to be able to bring in all possible dimension values, and not coalescing means that we miss out on cases with null values for one set of metrics, but non-null values for a different set of metrics.

Test Plan

Locally + unit tests

• PR has an associated issue: For metrics SQL generation, we should COALESCE dimensions after doing the FULL JOIN #1204
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

[netlify]

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