SOLR-18400: Plugins screen 500s when metrics are disabled

[janhoy]

https://issues.apache.org/jira/browse/SOLR-18400

When metrics collection is disabled, MetricsHandler adds an error entry instead of metrics, and PrometheusResponseWriter threw IOException("No metrics found in response") → HTTP 500 and a blank Plugins screen.

• PrometheusResponseWriter now writes the error as a Prometheus comment (# metrics collection is disabled) with HTTP 200 — matching the old JSON-era behavior and keeping the response a valid exposition
• Plugins screen shows an explanatory message instead of a blank page
• Regression test added to TestPrometheusResponseWriter (pure unit test, no cluster needed)

Verified with the Selenium Admin UI tests (SmokeTest + CollectionScreensTest, 11 tests) from the admin-ui-selenium-tests branch merged locally, and gradlew precommit.

[Copilot]

Pull request overview

Fixes the Plugins screen failure when Solr metrics collection is disabled by returning an explanatory response and UI message.

Changes:

• Handles disabled metrics in Prometheus responses.
• Displays an explanatory Plugins screen message.
• Adds regression coverage and 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	Summary
solr/webapp/web/partials/plugins.html	Displays the disabled-metrics message; configuration guidance needs correction.
solr/webapp/web/js/angular/controllers/plugins.js	Detects the disabled-metrics response.
solr/core/src/test/org/apache/solr/response/TestPrometheusResponseWriter.java	Adds regression coverage for the explanatory response.
solr/core/src/java/org/apache/solr/response/PrometheusResponseWriter.java	Serializes disabled metrics as comments; OpenMetrics responses also need # EOF.
changelog/unreleased/SOLR-18400-plugins-metrics-disabled.yml	Documents the fix.

Suppressed comments (1)

solr/webapp/web/js/angular/controllers/plugins.js:38

• This searches the entire exposition for the sentinel, so a valid metric HELP/comment/label containing this phrase will make the Plugins screen incorrectly report that metrics are disabled. Compare the complete error comment instead of using a substring match.

                $scope.metricsDisabled = (response.data || '').indexOf('metrics collection is disabled') !== -1;

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[janhoy]

Note: the v2 metrics API (GetMetrics) still throws a 510 INVALID_STATE error when metrics collection is disabled, so v1 and v2 now behave differently (v1 returns 200 with a # metrics collection is disabled comment). I left v2 as-is since this PR targets the v1 endpoint the Admin UI uses, but we may want to align v2 with the same graceful behavior in a follow-up.

[epugh]

Note: the v2 metrics API (GetMetrics) still throws a 510 INVALID_STATE error when metrics collection is disabled, so v1 and v2 now behave differently (v1 returns 200 with a # metrics collection is disabled comment). I left v2 as-is since this PR targets the v1 endpoint the Admin UI uses, but we may want to align v2 with the same graceful behavior in a follow-up.

It would make life easier when we move if V2 did the same as V1... One reason I'm axinous to get us to V2 everywhere we can is that we've seen this pattern of fixes making it to V1 when we also have V2, and then it falls behind...