SOLR-18198 Add support for "missing" stats in rollup for streaming expressions

[KhushJain]

https://issues.apache.org/jira/browse/SOLR-18198

Description

Rollup function to support missing statistics in /stream handler.

Solution

Add a new MissingMetric to the streaming expressions framework:

1. MissingMetric.java: New metric class modeled after CountMetric. Increments a counter when tuple.get(columnName) == null. Registered as function name missing.
2. Lang.java: Register missing function name.
3. ParallelMetricsRollup.java : Added SumMetric aggregation.

Tests

Updated existing tests in StreamingTest.java:

• Fixture data: Added a new field b_f to only 5 of the 10 documents in helloDocsUpdateRequest (ids 0, 4, 5, 6, 9), leaving the other 5 without it. This avoids changing existing a_f
  assertions.
• testRollupStream and testParallelRollupStream: Added MissingMetric("b_f") to the metrics array and asserted.

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

[Copilot]

Pull request overview

Adds a new missing(field) metric to SolrJ streaming expressions and ensures it can be correctly rolled up when metrics are computed in parallel/tiered contexts.

Changes:

• Introduces MissingMetric (new metric function missing) for counting null/missing values.
• Registers the new metric in Lang and adds rollup support in ParallelMetricsRollup.
• Updates streaming rollup tests and adds a changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
solr/solrj-streaming/src/test/org/apache/solr/client/solrj/io/stream/StreamingTest.java	Extends rollup/parallel rollup tests and fixtures to validate missing(b_f) behavior.
solr/solrj-streaming/src/java/org/apache/solr/client/solrj/io/stream/metrics/MissingMetric.java	Adds the new MissingMetric implementation and streaming-expression parsing/toExpression support.
solr/solrj-streaming/src/java/org/apache/solr/client/solrj/io/stream/ParallelMetricsRollup.java	Adds rollup aggregation logic for MissingMetric (sum across tiers).
solr/solrj-streaming/src/java/org/apache/solr/client/solrj/io/Lang.java	Registers missing as a streaming expression metric function.
changelog/unreleased/SOLR-18198-support-missing-stats-count-in-rollup.yml	Documents the feature addition in the changelog.

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[epugh]

Very close!

[epugh]

Can you update the docs as well???

[KhushJain]

Can you update the docs as well???

Yes, updated the doc.

[epugh]

LGTM pending the checks all running to completion!

[KhushJain]

Thanks @epugh! Can you backport to 10.x and 9.x?

[epugh]

Looks like one test failed? Can you investigate?

[KhushJain]

Looks like one test failed? Can you investigate?

Looks like the Lang test needed to include the new metric that was added. Let's try now @epugh