release-22.2.17-rc: sql/stats: don't use linear regression with NaN for stats forecasting

[DrewKimball]

Backport 2/2 commits from #113712.

/cc @cockroachdb/release

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This patch adds a method to the quantile function, invalid, which checks for NaN values within the function. These indicate a failure to derive a linear regression model for the observed histograms, and can lead to internal errors and panics if not caught. Stats forecasting now checks this method before attempting to use a quantile function to derive a histogram prediction.

Fixes #113645
Fixes #113680

Release note (bug fix): Fixed a bug that could cause internal errors or panics while attempting to forecast statistics on a numeric column.

Release justification: fix for internal error encountered by customer

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
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  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[michae2]

Reviewed 3 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball and @rafiss)

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pkg/sql/stats/stats_cache.go line 243 at r2 (raw file):

      }
    }
  }()

Looks like indentation is off here (I think this is what TestLint/TestGofmtSimplify is catching).

[DrewKimball]

Are all of the changes protected via a flag or option, new syntax, an environment variable or default-disabled cluster setting?

No, but this is a simple change that we are confident is correct.

What are the risks to backporting this change? Can the risks of backporting be mitigated?

If the change introduced a bug, it could make a table or tables effectively unavailable. However, as mentioned, we are confident that this is not the case.

What are the risks to not backporting?

Customers that run into the bug will find that a table is unavailable due to a NPE. This is rare, but has already happened in a few cases.

Does this change really need to be backported? Or can it reasonably wait until the next major release to be addressed?

It would be best to backport to active releases, since table unavailability is fairly severe and the best workaround currently is to disable stats forecasting for the affected table.

[DrewKimball]

@rafi friendly ping - this backport has been approved and is a blocker.