core: normalize service config number values (grpc#12826)

This updates default service config validation to accept numeric values
represented as `Number`, not only `Double`.

Common JSON parsers may deserialize integer-looking JSON values such as
`maxAttempts: 4` and `backoffMultiplier: 2` as `Integer`, which
previously caused `defaultServiceConfig()` to fail with
`IllegalArgumentException`.

The values are normalized to `Double` when copied into the validated
service config, preserving the existing internal representation expected
by the service config parsing code.

Fixes grpc#12815

Author: codingkiddo

core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

@@ -582,8 +582,8 @@ public ManagedChannelImplBuilder defaultServiceConfig(@Nullable Map<String, ?> s
         parsedMap.put(key, checkListEntryTypes((List<?>) value));
       } else if (value instanceof String) {
         parsedMap.put(key, value);
-      } else if (value instanceof Double) {
-        parsedMap.put(key, value);
+      } else if (value instanceof Number) {
+        parsedMap.put(key, ((Number) value).doubleValue());
       } else if (value instanceof Boolean) {
         parsedMap.put(key, value);
       } else {
@@ -606,8 +606,8 @@ private static List<?> checkListEntryTypes(List<?> list) {
         parsedList.add(checkListEntryTypes((List<?>) value));
       } else if (value instanceof String) {
         parsedList.add(value);
-      } else if (value instanceof Double) {
-        parsedList.add(value);
+      } else if (value instanceof Number) {
+        parsedList.add(((Number) value).doubleValue());
       } else if (value instanceof Boolean) {
         parsedList.add(value);
       } else {

core/src/test/java/io/grpc/internal/ManagedChannelImplBuilderTest.java

@@ -735,9 +735,31 @@ public void defaultServiceConfig_intKey() {
   @Test
   public void defaultServiceConfig_intValue() {
     Map<String, Object> config = new HashMap<>();
+
     config.put("key", 3);
 
-    assertThrows(IllegalArgumentException.class, () -> builder.defaultServiceConfig(config));
+    builder.defaultServiceConfig(config);
+
+    assertThat(builder.defaultServiceConfig).containsEntry("key", 3D);
+  }
+
+  @Test
+  public void defaultServiceConfig_nestedIntValue() {
+    Map<String, Object> config = new HashMap<>();
+    List<Object> list = new ArrayList<>();
+    Map<String, Object> nested = new HashMap<>();
+
+    list.add(123);
+    nested.put("key", 456);
+    list.add(nested);
+    config.put("list", list);
+
+    builder.defaultServiceConfig(config);
+    
+    assertThat(builder.defaultServiceConfig)
+            .containsEntry(
+                    "list",
+                    Arrays.asList(123D, Collections.singletonMap("key", 456D)));
   }
 
   @Test