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]

Could "" ever be a value?

[KhushJain]

"" is a valid value for a field. tuple.get(columnName) would only be null when the field doesn't exist. Missing counts the number of documents that doesn't have the field itself. The behavior is consistent with how stats component computes missing.

[epugh]

Can you update the docs as well???

[KhushJain]

Can you update the docs as well???

Yes, updated the doc.