Add hidden flag to set fetch parallelism for Exchange from CLI (#2879)

#### Does this PR need a docs update or release note?

- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* closes #2877

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - OneDrive item downloads that return 404 during backup (normally due to external deletion while Corso processes) are now skipped instead of quietly dropped.  These items will appear in the skipped list alongside other skipped cases such as malware detection.
 - Listing a single backup by id will also list the skipped and failed items that occurred during the backup.  These can be filtered out with the flags `--failed-items hide`, `--skipped-items hide`, and `--recovered-errors hide`.
 - Enable incremental backups for OneDrive if permissions aren't being backed up.
+- Hidden flag to control parallelism for fetching Exchange items (`--fetch-parallelism`). May help reduce `ApplicationThrottled` errors but will slow down backup.
 
 ### Fixed
 - Fix repo connect not working without a config file

src/cli/backup/exchange.go

@@ -117,6 +117,7 @@ func addExchangeCommands(cmd *cobra.Command) *cobra.Command {
 			&exchangeData,
 			utils.DataFN, nil,
 			"Select one or more types of data to backup: "+dataEmail+", "+dataContacts+", or "+dataEvents)
+		options.AddFetchParallelismFlag(c)
 		options.AddOperationFlags(c)
 
 	case listCommand:

src/cli/options/options.go

@@ -17,6 +17,7 @@ func Control() control.Options {
 	opt.SkipReduce = skipReduce
 	opt.ToggleFeatures.DisableIncrementals = disableIncrementals
 	opt.ToggleFeatures.EnablePermissionsBackup = enablePermissionsBackup
+	opt.ItemFetchParallelism = fetchParallelism
 
 	return opt
 }
@@ -30,6 +31,7 @@ var (
 	noStats            bool
 	restorePermissions bool
 	skipReduce         bool
+	fetchParallelism   int
 )
 
 // AddOperationFlags adds command-local operation flags
@@ -62,6 +64,16 @@ func AddSkipReduceFlag(cmd *cobra.Command) {
 	cobra.CheckErr(fs.MarkHidden("skip-reduce"))
 }
 
+func AddFetchParallelismFlag(cmd *cobra.Command) {
+	fs := cmd.Flags()
+	fs.IntVar(
+		&fetchParallelism,
+		"fetch-parallelism",
+		4,
+		"Control the number of concurrent data fetches for Exchange. Valid range is [1-4]. Default: 4")
+	cobra.CheckErr(fs.MarkHidden("fetch-parallelism"))
+}
+
 // ---------------------------------------------------------------------------
 // Feature Flags
 // ---------------------------------------------------------------------------
@@ -82,7 +94,7 @@ func AddFeatureToggle(cmd *cobra.Command, effs ...exposeFeatureFlag) {
 	}
 }
 
-// Adds the hidden '--no-incrementals' cli flag which, when set, disables
+// Adds the hidden '--disable-incrementals' cli flag which, when set, disables
 // incremental backups.
 func DisableIncrementals() func(*pflag.FlagSet) {
 	return func(fs *pflag.FlagSet) {