feat: support detailed metric config with backward compatibility (#510)

Author: dkrutskikh

lib/src/analyzers/lint_analyzer/metrics/metric_utils.dart

@@ -16,7 +16,11 @@ T? readNullableThreshold<T extends num>(
   Map<String, Object?> config,
   String metricId,
 ) {
-  final configValue = config[metricId]?.toString();
+  final metricConfig = config[metricId];
+
+  final configValue = metricConfig is Map<String, Object?>
+      ? metricConfig['threshold']?.toString()
+      : metricConfig?.toString();
 
   if (configValue != null && T == int) {
     return int.tryParse(configValue) as T?;

test/analyzers/lint_analyzer/metrics/metric_utils_test.dart

@@ -33,7 +33,7 @@ void main() {
 
       const _config = {
         metricId1: '$metricId1Value',
-        metricId2: '$metricId2Value',
+        metricId2: {'threshold': '$metricId2Value'},
         metricId3: '',
         metricId4: null,
       };