Avoid running SamplingServiceTests if sampling feature flag is missing (#135858)

The SamplingServiceTests fail when running in non-snapshot mode. This is
because we do not do any sampling if the sampling feature flag is not
enabled. This adds an assumption to all of those tests that the feature
flag is enabled. It also fixes a potential NullPointerException that
happens if you attempt to get the sampling service stats with the
sampling feature flag disabled. Closes #135808  Closes #135809 Closes
#135810 Closes #135811

Author: masseyke

muted-tests.yml

@@ -609,18 +609,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.qa.single_node.GenerativeIT
   method: test
   issue: https://github.com/elastic/elasticsearch/issues/135787
-- class: org.elasticsearch.ingest.SamplingServiceTests
-  method: testMaybeSampleWithLowRate
-  issue: https://github.com/elastic/elasticsearch/issues/135808
-- class: org.elasticsearch.ingest.SamplingServiceTests
-  method: testMaybeSample
-  issue: https://github.com/elastic/elasticsearch/issues/135809
-- class: org.elasticsearch.ingest.SamplingServiceTests
-  method: testMaybeSampleWithCondition
-  issue: https://github.com/elastic/elasticsearch/issues/135810
-- class: org.elasticsearch.ingest.SamplingServiceTests
-  method: testMaybeSampleMaxSamples
-  issue: https://github.com/elastic/elasticsearch/issues/135811
 - class: org.elasticsearch.xpack.esql.expression.function.aggregate.IrateTests
   method: testGroupingAggregate {TestCase=<positive ints>}
   issue: https://github.com/elastic/elasticsearch/issues/135814

server/src/main/java/org/elasticsearch/ingest/SamplingService.java

@@ -232,6 +232,9 @@ public List<RawDocument> getLocalSample(ProjectId projectId, String index) {
      */
     public SampleStats getLocalSampleStats(ProjectId projectId, String index) {
         SoftReference<SampleInfo> sampleInfoReference = samples.get(new ProjectIndex(projectId, index));
+        if (sampleInfoReference == null) {
+            return new SampleStats();
+        }
         SampleInfo sampleInfo = sampleInfoReference.get();
         return sampleInfo == null ? new SampleStats() : sampleInfo.stats;
     }

server/src/test/java/org/elasticsearch/ingest/SamplingServiceTests.java

@@ -34,18 +34,21 @@
 import java.util.List;
 import java.util.Map;
 
+import static org.elasticsearch.ingest.SamplingService.RANDOM_SAMPLING_FEATURE_FLAG;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.lessThan;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.nullValue;
+import static org.junit.Assume.assumeTrue;
 
 public class SamplingServiceTests extends ESTestCase {
 
     private static final String TEST_CONDITIONAL_SCRIPT = "ctx?.foo == 'bar'";
 
     public void testMaybeSample() {
+        assumeTrue("Requires sampling feature flag", RANDOM_SAMPLING_FEATURE_FLAG);
         SamplingService samplingService = getTestSamplingService();
 
         // First, test with a project that has no sampling config:
@@ -101,6 +104,7 @@ public void testMaybeSample() {
     }
 
     public void testMaybeSampleWithCondition() {
+        assumeTrue("Requires sampling feature flag", RANDOM_SAMPLING_FEATURE_FLAG);
         SamplingService samplingService = getTestSamplingService();
         String indexName = randomIdentifier();
         ProjectMetadata.Builder projectBuilder = ProjectMetadata.builder(ProjectId.DEFAULT)
@@ -147,6 +151,7 @@ public void testMaybeSampleWithCondition() {
     }
 
     public void testMaybeSampleWithLowRate() {
+        assumeTrue("Requires sampling feature flag", RANDOM_SAMPLING_FEATURE_FLAG);
         SamplingService samplingService = getTestSamplingService();
         String indexName = randomIdentifier();
         ProjectMetadata.Builder projectBuilder = ProjectMetadata.builder(ProjectId.DEFAULT)
@@ -185,6 +190,7 @@ public void testMaybeSampleWithLowRate() {
     }
 
     public void testMaybeSampleMaxSamples() {
+        assumeTrue("Requires sampling feature flag", RANDOM_SAMPLING_FEATURE_FLAG);
         SamplingService samplingService = getTestSamplingService();
         String indexName = randomIdentifier();
         int maxSamples = randomIntBetween(1, 1000);