Fix retrieving cube SQL failure

[shangyian]

Summary

There are two bugfixes in this PR:

(1) In some cases, retrieving cube SQL fails with this error:

greenlet_spawn has not been called; can't call await_only() here. Was IO attempted in an unexpected place?

This is because the node object loaded from the database did not have cube metadata on it pre-loaded, like cube_elements.

(2) We should refresh the metrics SQL cache every time it's requested. Otherwise when we deploy changes to the SQL build, we won't keep the cache refreshed with the changes.

Test Plan

Tested 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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