Fix ClassCastException in DebugMetrics nested structures

This commit resolves an issue where Double values in nested metric
structures were incorrectly cast to Map objects, causing a
ClassCastException. The fix allows for proper handling of both
direct values and nested structures at the same level.

A comprehensive test case has been added to reproduce the bug and
verify the fix, ensuring that complex, dynamically nested metric
structures can be handled without errors.

Resolves: hyperledger#7383
Signed-off-by: Ade Lucas <[email protected]>

ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/methods/DebugMetrics.java

@@ -71,7 +71,26 @@ private void addLabelledObservation(
   @SuppressWarnings("unchecked")
   private Map<String, Object> getNextMapLevel(
       final Map<String, Object> current, final String name) {
+    // Use compute to either return the existing map or create a new one
     return (Map<String, Object>)
-        current.computeIfAbsent(name, key -> new HashMap<String, Object>());
+        current.compute(
+            name,
+            (k, v) -> {
+              if (v instanceof Map) {
+                // If the value is already a Map, return it as is
+                return v;
+              } else {
+                // If the value is not a Map, create a new Map
+                Map<String, Object> newMap = new HashMap<>();
+                if (v != null) {
+                  // If v is not null and not a Map, we store it as a leaf value
+                  // If the original value was not null, store it under the "value" key
+                  // This handles cases where a metric value (e.g., Double) was previously stored
+                  // directly
+                  newMap.put("value", v);
+                }
+                return newMap;
+              }
+            });
   }
 }

ethereum/api/src/test/java/org/hyperledger/besu/ethereum/api/jsonrpc/internal/methods/DebugMetricsTest.java

@@ -16,6 +16,7 @@
 
 import static java.util.Arrays.asList;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.hyperledger.besu.metrics.BesuMetricCategory.BLOCKCHAIN;
 import static org.hyperledger.besu.metrics.BesuMetricCategory.PEERS;
 import static org.hyperledger.besu.metrics.BesuMetricCategory.RPC;
 import static org.mockito.Mockito.mock;
@@ -28,6 +29,7 @@
 import org.hyperledger.besu.metrics.Observation;
 
 import java.util.Collections;
+import java.util.List;
 import java.util.stream.Stream;
 
 import com.google.common.collect.ImmutableMap;
@@ -84,6 +86,38 @@ public void shouldNestObservationsByLabel() {
                     ImmutableMap.of("label2B", "value3")))));
   }
 
+  @Test
+  public void shouldHandleDoubleValuesInNestedStructureWithoutClassCastException() {
+    // Tests fix for issue# 7383: debug_metrics method error
+    when(metricsSystem.streamObservations())
+            .thenReturn(
+                    Stream.of(
+                            // This creates a double value for "a"
+                            new Observation(BLOCKCHAIN, "nested_metric", 1.0, List.of("a")),
+                            // This attempts to create a nested structure under "a", which was previously a double
+                            new Observation(BLOCKCHAIN, "nested_metric", 2.0, asList("a", "b")),
+                            // This adds another level of nesting
+                            new Observation(BLOCKCHAIN, "nested_metric", 3.0, asList("a", "b", "c"))
+                    ));
+
+    assertResponse(
+            ImmutableMap.of(
+                    BLOCKCHAIN.getName(),
+                    ImmutableMap.of(
+                            "nested_metric", ImmutableMap.of(
+                                    "a", ImmutableMap.of(
+                                            "value", 1.0,
+                                            "b", ImmutableMap.of(
+                                                    "value", 2.0,
+                                                    "c", 3.0
+                                            )
+                                    )
+                            )
+                    )
+            )
+    );
+  }
+
   private void assertResponse(final ImmutableMap<String, Object> expectedResponse) {
     final JsonRpcSuccessResponse response = (JsonRpcSuccessResponse) method.response(REQUEST);
     assertThat(response.getResult()).isEqualTo(expectedResponse);