Introduce Prometheus Java Client integration test (#1468)

And document some possible future improvements.

integration-tests/prometheus-java-client-expected-warnings.txt

@@ -0,0 +1,19 @@
+integration-tests/it-exporter/it-exporter-httpserver-sample/src/main/java/io/prometheus/metrics/it/exporter/httpserver/HTTPServerSample.java:[15,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'SUCCESS', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-httpserver-sample/src/main/java/io/prometheus/metrics/it/exporter/httpserver/HTTPServerSample.java:[16,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'ERROR', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-no-protobuf/src/main/java/io/prometheus/metrics/it/exporter/httpserver/HTTPServerSample.java:[15,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'SUCCESS', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-no-protobuf/src/main/java/io/prometheus/metrics/it/exporter/httpserver/HTTPServerSample.java:[16,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'ERROR', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-servlet-jetty-sample/src/main/java/io/prometheus/metrics/it/exporter/servlet/jetty/ExporterServletJettySample.java:[18,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'SUCCESS', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-servlet-jetty-sample/src/main/java/io/prometheus/metrics/it/exporter/servlet/jetty/ExporterServletJettySample.java:[19,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'ERROR', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-servlet-tomcat-sample/src/main/java/io/prometheus/metrics/it/exporter/servlet/tomcat/ExporterServletTomcatSample.java:[21,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'SUCCESS', though note that this is not a private constant
+integration-tests/it-exporter/it-exporter-servlet-tomcat-sample/src/main/java/io/prometheus/metrics/it/exporter/servlet/tomcat/ExporterServletTomcatSample.java:[22,5] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'ERROR', though note that this is not a private constant
+prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/CounterTest.java:[180,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that a method named `incWithExemplar` is already defined in this class or a supertype)
+prometheus-metrics-instrumentation-jvm/src/main/java/io/prometheus/metrics/instrumentation/jvm/JvmNativeMemoryMetrics.java:[96,30] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'IS_ENABLED', though note that this is not a private constant
+prometheus-metrics-instrumentation-jvm/src/main/java/io/prometheus/metrics/instrumentation/jvm/NativeImageChecker.java:[11,24] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'IS_GRAAL_VM_NATIVE_IMAGE', though note that this is not a private constant
+prometheus-metrics-model/src/main/java/io/prometheus/metrics/model/registry/PrometheusRegistry.java:[16,42] [ConstantNaming] Constant variables should adhere to the `UPPER_SNAKE_CASE` naming convention; consider renaming to 'DEFAULT_REGISTRY', though note that this is not a private constant
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/LabelTest.java:[19,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that a method named `toString` is already defined in this class or a supertype)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/LabelTest.java:[24,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that a method named `equals` is already defined in this class or a supertype)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/LabelTest.java:[29,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that a method named `hashCode` is already defined in this class or a supertype)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/PrometheusNamingTest.java:[12,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that another method named `sanitizeMetricName` is in scope)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/PrometheusNamingTest.java:[44,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that another method named `sanitizeLabelName` is in scope)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/PrometheusNamingTest.java:[55,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that another method named `validateUnitName` is in scope)
+prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/PrometheusNamingTest.java:[67,8] [JUnitMethodDeclaration] This method's name should not redundantly start with `test` (but note that another method named `sanitizeUnitName` is in scope)

integration-tests/prometheus-java-client-init.patch

@@ -0,0 +1,36 @@
+--- a/pom.xml
++++ b/pom.xml
+@@ -128,7 +128,6 @@
+         <dependency>
+             <groupId>com.google.guava</groupId>
+             <artifactId>guava</artifactId>
+-            <scope>test</scope>
+         </dependency>
+         <dependency>
+             <groupId>org.slf4j</groupId>
+@@ -370,13 +369,24 @@
+                             -Xep:LongDoubleConversion:OFF
+                             -Xep:StringSplitter:OFF
+                             -XepExcludedPaths:.*/generated/.*
++                            ${error-prone.configuration-args}
+                         </arg>
+                     </compilerArgs>
+                     <annotationProcessorPaths>
+                         <path>
+                             <groupId>com.google.errorprone</groupId>
+                             <artifactId>error_prone_core</artifactId>
+-                            <version>2.36.0</version>
++                            <version>${error-prone.version}</version>
++                        </path>
++                        <path>
++                            <groupId>tech.picnic.error-prone-support</groupId>
++                            <artifactId>error-prone-contrib</artifactId>
++                            <version>${error-prone-support.version}</version>
++                        </path>
++                        <path>
++                            <groupId>tech.picnic.error-prone-support</groupId>
++                            <artifactId>refaster-runner</artifactId>
++                            <version>${error-prone-support.version}</version>
+                         </path>
+                         <!-- Other annotation processors go here.
+ 

integration-tests/prometheus-java-client.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+set -e -u -o pipefail
+
+test_name="$(basename "${0}" .sh)"
+project='prometheus-java-client'
+repository='https://github.com/prometheus/client_java.git'
+revision='v1.3.5'
+additional_build_flags='-Djava.version=11 -Dwarnings='
+additional_source_directories=''
+shared_error_prone_flags=''
+patch_error_prone_flags=''
+validation_error_prone_flags=''
+# Validation skips some tests:
+# - Starting from a clean repository, the `PushGatewayIT` tests reference a JAR
+#   file that is created only after test completion, causing the tests to fail.
+# - The `SlidingWindowTest#rotate` test is flaky.
+# XXX: Drop the `SlidingWindowTest` exclusion once prometheus/client_java#1242
+# is resolved. For unclear reasons, it appears that without this exclusion the
+# integration tests are no longer executed, meaning that perhaps all flags
+# specified here can then be dropped.
+validation_build_flags='-Dtest=!PushGatewayIT,!SlidingWindowTest#rotate -Dsurefire.failIfNoSpecifiedTests=false'
+
+if [ "${#}" -gt 2 ] || ([ "${#}" = 2 ] && [ "${1:---sync}" != '--sync' ]); then
+  >&2 echo "Usage: ${0} [--sync] [<report_directory>]"
+  exit 1
+fi
+
+"$(dirname "${0}")/run-integration-test.sh" \
+  "${test_name}" \
+  "${project}" \
+  "${repository}" \
+  "${revision}" \
+  "${additional_build_flags}" \
+  "${additional_source_directories}" \
+  "${shared_error_prone_flags}" \
+  "${patch_error_prone_flags}" \
+  "${validation_error_prone_flags}" \
+  "${validation_build_flags}" \
+  $@