[FLINK-38704][metrics] Fix MetricConfig.getString() to handle numeric values

MetricConfig.getString() was using getProperty() which returns null for
numeric YAML values stored as Integer/Long/Boolean. Changed to use get()
and convert to String, matching the pattern of other getter methods.

This fixes metrics reporters not loading numeric configuration values.

Author: mukul-8

flink-metrics/flink-metrics-core/src/main/java/org/apache/flink/metrics/MetricConfig.java

@@ -34,7 +34,11 @@
 public class MetricConfig extends Properties {
 
     public String getString(String key, String defaultValue) {
-        return getProperty(key, defaultValue);
+        final Object value = get(key);
+        if (value == null) {
+            return defaultValue;
+        }
+        return value.toString();
     }
 
     /**

flink-metrics/flink-metrics-core/src/test/java/org/apache/flink/metrics/MetricConfigTest.java

@@ -90,7 +90,14 @@ private static Stream<Arguments> nativeTypeCases() {
                         123456789012345L),
                 Arguments.of(3.14f, (TypedGetter) (c, k) -> c.getFloat(k, 0.0f), 3.14f),
                 Arguments.of(2.718281828, (TypedGetter) (c, k) -> c.getDouble(k, 0.0), 2.718281828),
-                Arguments.of(true, (TypedGetter) (c, k) -> c.getBoolean(k, false), true));
+                Arguments.of(true, (TypedGetter) (c, k) -> c.getBoolean(k, false), true),
+                Arguments.of(42, (TypedGetter) (c, k) -> c.getString(k, "default"), "42"),
+                Arguments.of(
+                        123456789012345L,
+                        (TypedGetter) (c, k) -> c.getString(k, "default"),
+                        "123456789012345"),
+                Arguments.of(3.14f, (TypedGetter) (c, k) -> c.getString(k, "default"), "3.14"),
+                Arguments.of(true, (TypedGetter) (c, k) -> c.getString(k, "default"), "true"));
     }
 
     private static Stream<Arguments> crossTypeCases() {