[Lens as code] Merge metrics into a single property for Metric chart

src/platform/packages/shared/kbn-lens-embeddable-utils/config_builder/schema/bucket_ops.ts

@@ -194,11 +194,11 @@ export const bucketTermsOperationSchema = schema.object(
         schema.object({
           type: schema.literal('column'),
           /**
-           * Metric to be used for the column
+           * Metric to be used for the column by index number (0 based)
            */
           metric: schema.number({
             meta: {
-              description: 'Metric to be used for the column',
+              description: 'Metric to be used for the column by index number (0 based)',
             },
           }),
           /**