Better input validation on get_sql_for_metrics().

[agorajek]

Summary

We did not properly check the input to /sql api. This PR does a better job, example:

1. When metrics arg has a non-existing node:
   • before: 'NoneType' object has no attribute 'current'
   • after: datajunction_server.errors.DJNodeNotFound: A node with namefoo.bar.bazdoes not exist.
2. When metrics argument has a non-metric node:
   • before: {"message":"","errors":[{"code":204,"message":"","debug":null,"context":""}],"warnings":[]}
   • after: datajunction_server.errors.DJInvalidInputException: All nodes must be of metric type, but some are not: system.logs.cube (cube).

Test Plan

Locally.

• [x] PR has an associated issue: #1226
• [x] make check passes
• [x] make test shows 100% unit test coverage

Deployment Plan

auto

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[agorajek]

Is that error 'NoneType' object has no attribute 'current' coming from here? https://github.com/DataJunction/dj/blob/main/datajunction-server/datajunction_server/api/helpers.py#L365-L380

Nope. It was coming from here: https://github.com/DataJunction/dj/blob/main/datajunction-server/datajunction_server/database/queryrequest.py#L313

[samredai]

Is that error 'NoneType' object has no attribute 'current' coming from here? https://github.com/DataJunction/dj/blob/main/datajunction-server/datajunction_server/api/helpers.py#L365-L380

Nope. It was coming from here: https://github.com/DataJunction/dj/blob/main/datajunction-server/datajunction_server/database/queryrequest.py#L313

I see. I don't want to hold this PR up but I'm worried about having to perform the query for the metric nodes twice--once to make sure they exist, and then again when we're generating the sql. I suspect there's a way to just make the second time we call it robust enough to raise the node not found error like you did here.