Get collection usage size with ARM metrics call (#327)

- Removed `readCollectionQuotaInfo` call. The only data we need is the usage size which we can get via the ARM metrics call instead.
- Added `getCollectionUsageSize` which fetches the `DataUsage` and `IndexUsage` metrics, converts them to KB, and returns the sum as the total usage size

src/Explorer/Controls/Settings/TestUtils.tsx

@@ -18,7 +18,7 @@ export const collection = ({
     excludedPaths: []
   }),
   uniqueKeyPolicy: {} as DataModels.UniqueKeyPolicy,
-  quotaInfo: ko.observable<DataModels.CollectionQuotaInfo>({} as DataModels.CollectionQuotaInfo),
+  usageSizeInKB: ko.observable(100),
   offer: ko.observable<DataModels.Offer>({
     autoscaleMaxThroughput: undefined,
     manualThroughput: 10000,