SOLR-18328 Add support for standard deviation in rollup for streaming expressions

[KhushJain]

https://issues.apache.org/jira/browse/SOLR-18328

Description

Added support for the std(col) (standard deviation) metric in rollup() streaming expressions.
The std(col) metric was non-functional; the StdMetric class was a stub copy-pasted from MeanMetric — update() was a no-op, getValue() returned null, and newInstance() even returned a MeanMetric instead of a StdMetric.

Solution

Implemented StdMetric to compute the sample standard deviation:

StdMetric.java:

1. update() now accumulates count, sum, and sumSq from the column value (handling Double/Float/Integer/Long, skipping nulls/other types).
2. getValue() computes the sample standard deviation sqrt((n*sumSq - sum^2) / (n*(n-1))), returning 0.0 when count <= 1, and honoring outputLong (rounds to a long when set).
3. Fixed newInstance() to return a StdMetric (was incorrectly returning MeanMetric), and replaced the unused doubleSum/longSum fields with sum/sumSq.

Tests

• StreamExpressionTest.testRollupStdMetric (new): validates std(a_i) and std(a_f) alongside count(*) in a rollup(... over="a_s" ...) expression across multiple groups.
• StreamingTest.testRollupStream: added StdMetric("a_i") / StdMetric("a_f") to the metrics array and asserted expected values per group.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change

[KhushJain]

Hey @epugh, thanks for reviewing!!

One of the existing flaky test failed testSingleShardInFlightRequestsDuringShutDown

[epugh]

a question about docs, but otherwise sems to follow the pattern of some other ones.. It would be good to answer the other review comments, and tjhen I think we can move forward...

[KhushJain]

Thanks @epugh @VishnuPriyaChandraSekar for reviewing it!!

Can we please back port this to 9x as well?

[VishnuPriyaChandraSekar]

Thanks for addressing my feedback :)

[KhushJain]

Thanks for reviewing!!
Could one of you merge and back port to 9x please @epugh @VishnuPriyaChandraSekar

[epugh]

LGTM

[KhushJain]

@epugh will this be part of the 9.11 release?
Any idea when we’ll release 9.11, since the release was previously stopped?

[epugh]

argh... so, in my backporrt, I got confused and I thought backport burped on lciense, but instead it's failing on java code pattern...

[epugh]

FYI: the 9x support is in #4723

[epugh]

@epugh will this be part of the 9.11 release? Any idea when we’ll release 9.11, since the release was previously stopped?

There has been discussion about getting 9.11 out this month.