Use md as default modal size

app/assets/javascripts/performance_bar/components/detailed_metric.vue

@@ -56,7 +56,7 @@ export default {
     <gl-modal
       :id="`modal-peek-${metric}-details`"
       :header-title-text="header"
-      modal-size="modal-lg"
+      modal-size="lg"
       class="performance-bar-modal"
     >
       <table

app/assets/javascripts/vue_shared/components/gl_modal.vue

@@ -13,7 +13,7 @@ export default {
     modalSize: {
       type: String,
       required: false,
-      default: '',
+      default: 'md',
     },
     headerTitleText: {
       type: String,
@@ -53,7 +53,7 @@ export default {
   >
     <div
       class="modal-dialog"
-      :class="modalSize"
+      :class="`modal-${modalSize}`"
       role="document"
     >
       <div class="modal-content">