ci: replace Maven Checkstyle with Flint (#2341)

## Summary

- use the released Flint 0.22.10 Checkstyle implementation
- add Checkstyle 13.8.0 as the direct `checkstyle` mise entry
- update CI to mise 2026.7.15 so that direct registry entry is available
in every workflow
- allow the verified `@yarnpkg/libzip` 3.2.2 release for Renovate
because it was published without provenance
- remove the Maven Checkstyle plugin and its obsolete skip settings so
Checkstyle runs only once, through Flint
- preserve the repository's previous main-source scope with narrow
suppressions for test and archived Java sources
- suppress `VariableDeclarationUsageDistance` for examples, where setup
declarations intentionally precede simulated work

Flint Checkstyle support is released in
[v0.22.10](https://github.com/grafana/flint/releases/tag/v0.22.10), and
this PR uses the standard `aqua:grafana/flint` backend for Flint itself.

## Testing

- `mise exec -- flint run --full checkstyle`
- `mise run lint:fix`
- `mise run build`

---------

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.github/renovate-tracked-deps.json

@@ -163,6 +163,7 @@
         "aqua:jonwiggins/xmloxide",
         "aqua:owenlamont/ryl",
         "biome",
+        "checkstyle",
         "editorconfig-checker",
         "google-java-format",
         "hugo",

AGENTS.md

@@ -15,23 +15,23 @@ mise run ci
 # Quick compile without tests or checks (fastest)
 mise run compile
 
-# Run unit tests only (skips formatting/coverage/checkstyle)
+# Run unit tests only (skips formatting and coverage)
 mise run test
 
 # Run all tests including integration tests
 mise run test-all
 
 # Run a single test class
 ./mvnw test -Dtest=CounterTest \
-  -Dcoverage.skip=true -Dcheckstyle.skip=true
+  -Dcoverage.skip=true
 
 # Run a single test method
 ./mvnw test -Dtest=CounterTest#testIncrement \
-  -Dcoverage.skip=true -Dcheckstyle.skip=true
+  -Dcoverage.skip=true
 
 # Run tests in a specific module
 ./mvnw test -pl prometheus-metrics-core \
-  -Dcoverage.skip=true -Dcheckstyle.skip=true
+  -Dcoverage.skip=true
 
 # Regenerate protobuf classes (after protobuf dep update)
 mise run generate

CONTRIBUTING.md

@@ -96,14 +96,12 @@ mise --cd .mise/envs/native run native-test
 ### Avoid failures while running tests
 
 - Use `-Dcoverage.skip=true` to skip the coverage check during development.
-- Use `-Dcheckstyle.skip=true` to skip the checkstyle check during development.
 - Use `-Dwarnings=-nowarn` to skip the warnings during development.
 
 Combine all with
 
 ```shell
-./mvnw install -DskipTests -Dcoverage.skip=true \
-  -Dcheckstyle.skip=true -Dwarnings=-nowarn
+./mvnw install -DskipTests -Dcoverage.skip=true -Dwarnings=-nowarn
 ```
 
 or simply

checkstyle-suppressions.xml

@@ -21,4 +21,12 @@
 
   <!-- Suppress all Javadoc checks for integration tests -->
   <suppress checks="Javadoc" files=".*[/\\]integration-tests[/\\].*"/>
+
+  <!-- Preserve the repository's main-source-only Checkstyle policy when Flint invokes the
+       standalone CLI with explicit Java files. -->
+  <suppress checks=".*" files=".*[/\\]src[/\\]test[/\\]java[/\\].*"/>
+  <suppress checks=".*" files=".*[/\\]src[/\\]archive[/\\]java[/\\].*"/>
+
+  <!-- Example setup intentionally groups declarations before the simulated work that uses them. -->
+  <suppress checks="VariableDeclarationUsageDistance" files=".*[/\\]examples[/\\].*"/>
 </suppressions>

checkstyle.xml

@@ -48,18 +48,6 @@
   </module>
 
   <module name="TreeWalker">
-    <module name="RegexpSinglelineJava">
-      <property name="format"
-                value="(?&lt;!import static io.opentelemetry.sdk.testing.assertj.)OpenTelemetryAssertions\."/>
-      <property name="message"
-                value="Please statically import methods from OpenTelemetryAssertions"/>
-    </module>
-    <module name="RegexpSinglelineJava">
-      <property name="format"
-                value="(?&lt;!import static org.assertj.core.api.|OpenTelemetry)Assertions.assertThat"/>
-      <property name="message"
-                value="Please statically import methods from Assertions (OpenTelemetryAssertions extends Assertions)"/>
-    </module>
     <module name="OuterTypeFilename"/>
     <module name="IllegalTokenText">
       <property name="tokens" value="STRING_LITERAL, CHAR_LITERAL"/>

mise.lock

@@ -12,6 +12,7 @@ actionlint = "1.7.12"
 "aqua:jonwiggins/xmloxide" = "v0.5.0"
 "aqua:owenlamont/ryl" = "0.21.0"
 biome = "2.5.8"
+checkstyle = "13.8.0"
 editorconfig-checker = "3.11.1"
 google-java-format = "1.36.1"
 lychee = "0.24.2"
@@ -48,16 +49,16 @@ run = "./mvnw clean"
 
 [tasks.compile]
 description = "bare compile, ignoring formatting and linters"
-run = "./mvnw install -DskipTests -Dcoverage.skip=true -Dcheckstyle.skip=true -Dwarnings=-nowarn"
+run = "./mvnw install -DskipTests -Dcoverage.skip=true -Dwarnings=-nowarn"
 
 [tasks.generate]
 description = "regenerate protobuf sources"
-run = "./mvnw clean install -DskipTests -Dcoverage.skip=true -Dcheckstyle.skip=true -Dwarnings=-nowarn"
+run = "./mvnw clean install -DskipTests -Dcoverage.skip=true -Dwarnings=-nowarn"
 env.PROTO_GENERATION = "true"
 
 [tasks.test]
 description = "run unit tests, ignoring formatting and linters"
-run = "./mvnw test -Dcoverage.skip=true -Dcheckstyle.skip=true -Dwarnings=-nowarn"
+run = "./mvnw test -Dcoverage.skip=true -Dwarnings=-nowarn"
 
 [tasks.test-all]
 description = "run all tests"
@@ -78,7 +79,6 @@ BASELINE_OVERRIDE="${API_DIFF_BASELINE_VERSION:+-Dapi.diff.baseline.version=${AP
   ${BASELINE_OVERRIDE} \
   -DskipTests \
   -Dcoverage.skip=true \
-  -Dcheckstyle.skip=true \
   -Dwarnings=-nowarn
 ./.github/scripts/sync-api-diffs.sh
 """

mise.toml

@@ -32,7 +32,6 @@
     <!-- Bumped by .github/workflows/bump-api-diff-baseline.yml after each release. -->
     <api.diff.baseline.version>1.8.0</api.diff.baseline.version>
     <jacoco.line-coverage>0.70</jacoco.line-coverage>
-    <checkstyle.skip>false</checkstyle.skip>
     <coverage.skip>false</coverage.skip>
     <javadoc.skip>false</javadoc.skip>
     <warnings>-Werror</warnings>
@@ -182,28 +181,6 @@
       </plugins>
     </pluginManagement>
     <plugins>
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-checkstyle-plugin</artifactId>
-        <version>3.6.0</version>
-        <configuration>
-          <consoleOutput>true</consoleOutput>
-          <configLocation>google_checks.xml</configLocation>
-          <configLocation>checkstyle.xml</configLocation>
-          <violationSeverity>warning</violationSeverity>
-          <failOnViolation>true</failOnViolation>
-          <skip>${checkstyle.skip}</skip>
-          <suppressionsLocation>checkstyle-suppressions.xml</suppressionsLocation>
-          <excludes>**/generated/**,**/jmh_generated/*</excludes>
-        </configuration>
-        <executions>
-          <execution>
-            <goals>
-              <goal>check</goal>
-            </goals>
-          </execution>
-        </executions>
-      </plugin>
       <plugin>
         <groupId>org.jacoco</groupId>
         <artifactId>jacoco-maven-plugin</artifactId>

pom.xml

@@ -19,7 +19,6 @@
   </description>
 
   <properties>
-    <checkstyle.skip>true</checkstyle.skip>
     <coverage.skip>true</coverage.skip>
     <javadoc.skip>true</javadoc.skip>
   </properties>

prometheus-metrics-otel-support/pom.xml

@@ -14,7 +14,6 @@
   </description>
 
   <properties>
-    <checkstyle.skip>true</checkstyle.skip>
     <coverage.skip>true</coverage.skip>
     <javadoc.skip>true</javadoc.skip>
   </properties>