[BUG] Cannot set `User-Agent` header in track2 SDK

Library name and version

Azure.ResourceManager

Describe the bug

Description

The following code snippet (copied from https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/Azure.Core/samples/Pipeline.md#implementing-a-synchronous-policy) doesn’t work.

public class CustomRequestPolicy : HttpPipelineSynchronousPolicy
{
    public override void OnSendingRequest(HttpMessage message)
    {
        message.Request.Headers.Add("User-Agent", "abc");
    }
}

The root cause is that we have an internal override https://github.com/Azure/azure-sdk-for-net/blob/d269eede20a23edec101477601a2b90bee54e690/sdk/resourcemanager/Azure.ResourceManager/src/Internal/MgmtTelemetryPolicy.cs#L21 So external users cannot simply set User-Agent by Headers.Add() or Headers.Set().

Expected behavior

The following code snippet should add a User-Agent header value.

public class CustomRequestPolicy : HttpPipelineSynchronousPolicy
{
    public override void OnSendingRequest(HttpMessage message)
    {
        message.Request.Headers.Add("User-Agent", "abc");
    }
}

Actual behavior

User-Agent header value cannot be modified through Headers.Add() or Headers.Set().

Reproduction Steps

Try the code snippet at the beginning.

Environment

No response

[archerzz]

I think we need to make a decision: shall we allow users to modify User-Agent? We could change the logic in MgmtTelemetryPolicy.cs to add (v.s. set) the value passed by UserAgentOverride, which will make sure the User-Agent set by each SDK will be passed to back-end.

[archerzz]

I checked with Powershell team. And it looks like the backend analytics can support long strings. So appending custom user-agent headers seems the best option, e.g. the final header value will look like:

azsdk-net-ResourceManager.XXX/1.0.0-alpha.20220124.1 (.NET Framework 4.8.4420.0; Microsoft Windows 10.0.19043 ) AzurePowershell/v6.6.0 Az.KeyValue/4.1.0 PSVersion/v7.1.3