A lot of traces and dependencies items with operation_name MessageRelayOutbox in Application Insight

Environnent Information :
- FUNCTIONS_EXTENSION_VERSION : ~3
- FUNCTIONS_WORKER_RUNTIME : dotnet
- Function App version:
- Invocation ID: cd561f4e-dae7-4ead-976d-9eb7e7fca057
- Region: West Europe
- Development Language: C#

I am seeing a lot of traces and dependencies items with operation_name MessageRelayOutbox in Application Insight. For example in the weekend only a TimerTrigger is running purging the durable functions Instance and History table like:

```
private async Task ClearItemsAsync(IDurableClient client, DateTime fromDate, DateTime toDate, IEnumerable runtimeStatus)
{
this.logger.LogInformation($"Purging history and instance table for {runtimeStatus} items created between {fromDate:yyyy-MM-ddTHH:mm:ssZ} and {toDate:yyyy-MM-ddTHH:mm:ssZ}.");

PurgeHistoryResult purgeResult = await client.PurgeInstanceHistoryAsync(
fromDate,
toDate,
runtimeStatus).ConfigureAwait(true); // ConfigureAwait should be set to true in Azure functions.

this.logger.LogInformation($"Purged {purgeResult.InstancesDeleted} instances.");
}
```

The TimerTrigger contains a schedule expression like: “0 0 * * * *” (so it’s running every hour).

When I perform the following query in Application Insight:
```
dependencies
| where timestamp > datetime("2021-11-21T00:00:00.600Z") and timestamp < datetime("2021-11-21T23:00:00.600Z")
| summarize sum(_BilledSize) by operation_Name
```
I get the following result:
- MessageRelayOutbox: 26,545,680
- PurgeHistoryTimer: 29,716
Meaning a lot MessageRelayOutbox items, which feels strange to me.

The Timer function looks like:

```
private const string EveryHour = "0 0 * * * *";
private readonly ILogger logger;

///
/// Initializes a new instance of the class.
///
/// an instance of .
public PurgeHistoryTimer(ILogger logger)
{
this.logger = logger;
}

///
/// Called when the timer expires.
///
/// an instance of .
/// an instance of .
/// A representing the asynchronous operation.
[FunctionName(nameof(PurgeHistoryTimer))]
public async Task RunAsync(
[TimerTrigger(EveryHour)] TimerInfo timer,
[DurableClient] IDurableClient client)
{
timer.AssertIsNotNull(nameof(timer));
client.AssertIsNotNull(nameof(client));

await this.ClearCompletedItems(client).ConfigureAwait(false);

await this.ClearFailedItems(client).ConfigureAwait(false);
}
```

It looks like all the traces and dependencies are related to managed identity. Example messages returned from App Insights:
Columns
timestamp [UTC]
id
target
type
name
data
success
resultCode
duration
performanceBucket
itemType
customDimensions
operation_Name
operation_Id
operation_ParentId
client_Type
client_IP
cloud_RoleName
cloud_RoleInstance
appId

Example1
`11/22/2021, 10:55:39.997 AM 7dc2bb6e9d9b4843 127.0.0.1:41967 HTTP GET /MSI/token/ http://127.0.0.1:41967/MSI/token/?api-version=2017-09-01&resource=546d59fd-bd41-41a3-b700-780c50fc0162&clientid=a6b947bd-****-****-****-************ True 200 2.893 <250ms dependency {"Category":"Function.MessageRelayOutbox","ProcessId":"860","HostInstanceId":"c09a9f94-cbd1-4e79-a9b6-4f34d67e2063","ClientRequestId":"adb051a9-9627-4e14-bcb7-dfa65647020f","LogLevel":"Information","InvocationId":"cd561f4e-dae7-4ead-976d-9eb7e7fca057"} MessageRelayOutbox dc3d7afd221e0248aba00e4005f50555 2ce0cced6a2ff044 PC 0.0.0.0 func-XYZ-configurationmessagerelayoutbox-p01 dbd679437e659890e5b5d2b38d5d02495bbfed5e20ecb7205e78c1eca85ef87b d2748353-e09c-4eea-a08f-364d9b731086 `

Example2
`11/22/2021, 10:55:39.997 AM 1f90608cd0320340 ManagedIdentityCredential.GetToken InProc | Microsoft.AAD ManagedIdentityCredential.GetToken True 3.244 <250ms dependency {"Category":"Function.MessageRelayOutbox","ProcessId":"860","HostInstanceId":"c09a9f94-cbd1-4e79-a9b6-4f34d67e2063","LogLevel":"Information","InvocationId":"cd561f4e-dae7-4ead-976d-9eb7e7fca057"} MessageRelayOutbox dc3d7afd221e0248aba00e4005f50555 2ce0cced6a2ff044 PC 0.0.0.0 func-XYZ-configurationmessagerelayoutbox-p01 dbd679437e659890e5b5d2b38d5d02495bbfed5e20ecb7205e78c1eca85ef87b d2748353-e09c-4eea-a08f-364d9b731086 `

Example3
`11/22/2021, 10:55:39.997 AM 2ce0cced6a2ff044 DefaultAzureCredential.GetToken InProc | Microsoft.AAD DefaultAzureCredential.GetToken True 3.491 <250ms dependency {"Category":"Function.MessageRelayOutbox","ProcessId":"860","HostInstanceId":"c09a9f94-cbd1-4e79-a9b6-4f34d67e2063","LogLevel":"Information","InvocationId":"cd561f4e-dae7-4ead-976d-9eb7e7fca057"} MessageRelayOutbox dc3d7afd221e0248aba00e4005f50555 72d4a9c8e55e384e PC 0.0.0.0 func-XYZ-configurationmessagerelayoutbox-p01 dbd679437e659890e5b5d2b38d5d02495bbfed5e20ecb7205e78c1eca85ef87b d2748353-e09c-4eea-a08f-364d9b731086 `

Example4
`11/22/2021, 10:55:49.995 AM ccae72e5bcdade4a 127.0.0.1:41967 HTTP GET /MSI/token/ http://127.0.0.1:41967/MSI/token/?api-version=2017-09-01&resource=546d59fd-bd41-41a3-b700-780c50fc0162&clientid=a6b947bd-****-****-****-************ True 200 3.14 <250ms dependency {"Category":"Function.MessageRelayOutbox","ProcessId":"860","HostInstanceId":"c09a9f94-cbd1-4e79-a9b6-4f34d67e2063","ClientRequestId":"ddf9c4a7-7d26-495c-b2f4-33c221e3d589","LogLevel":"Information","InvocationId":"c381a384-def2-48e0-bbb2-67fad2dcf4ee"} MessageRelayOutbox 1b392970b56f914b9cc46f96f9fda2af 93f06c33f27d734a PC 0.0.0.0 func-XYZ-configurationmessagerelayoutbox-p01 dbd679437e659890e5b5d2b38d5d02495bbfed5e20ecb7205e78c1eca85ef87b d2748353-e09c-4eea-a08f-364d9b731086 `

Example5
`11/22/2021, 10:55:49.995 AM 9fcac7f61a011a42 ManagedIdentityCredential.GetToken InProc | Microsoft.AAD ManagedIdentityCredential.GetToken True 3.528 <250ms dependency {"Category":"Function.MessageRelayOutbox","ProcessId":"860","HostInstanceId":"c09a9f94-cbd1-4e79-a9b6-4f34d67e2063","LogLevel":"Information","InvocationId":"c381a384-def2-48e0-bbb2-67fad2dcf4ee"} MessageRelayOutbox 1b392970b56f914b9cc46f96f9fda2af 93f06c33f27d734a PC 0.0.0.0 func-XYZ-configurationmessagerelayoutbox-p01 dbd679437e659890e5b5d2b38d5d02495bbfed5e20ecb7205e78c1eca85ef87b d2748353-e09c-4eea-a08f-364d9b731086 `

Question: is amount of MessageRelayOutbox traces and dependencies items as expected or is something wrong?expected?