Suggested Cards: Add new feature flag to control the suggested cards feature (#6436)

## Motivation for features / changes
We are working on a new feature to suggest cards to a user based on
their previous interactions with TensorBoard.
This feature flag will help control it's rollout.

Author: rileyajones

tensorboard/webapp/feature_flag/store/feature_flag_metadata.ts

@@ -115,6 +115,11 @@ export const FeatureFlagMetadataMap: FeatureFlagMetadataMapType<FeatureFlags> =
       queryParamOverride: 'enableHparamsInTimeSeries',
       parseValue: parseBoolean,
     },
+    enableSuggestedCards: {
+      defaultValue: false,
+      queryParamOverride: 'enableSuggestedCards',
+      parseValue: parseBoolean,
+    },
   };
 
 /**

tensorboard/webapp/feature_flag/types.ts

@@ -47,4 +47,7 @@ export interface FeatureFlags {
   enableScalarColumnCustomization: boolean;
   // Adds hparam columns to the runs table and the scalar card data table.
   enableHparamsInTimeSeries: boolean;
+  // Adds a new section at the top of the time series metrics view
+  // containing suggested cards based on the users previous interactions.
+  enableSuggestedCards: boolean;
 }