ExpressRoute Traffic Collector, is a service that enables deep visibility into traffic within ExpressRoute Circuits. The Traffic Collector aggregates flow logs from ER circuits and forwards them to Azure tools such as Log Analytics for network traffic analysis, visualization, and insights. Depending on your termination of hardware on premises where visibility of this traffic may not be passed through an aggregation device which has sufficient logging capabilities, this allows you to achieve that visibility natively within Azure Log Analytics workspaces.

At the time of writing this article it is only available in the following regions, but it would be expected that this is rolled out to additional regions throughout the preview phase.

• Central US
• East US
• East US 2
• North Central US
• South Central US
• West Central US
• West US
• West US 2
• West US 3

For more information you can also view the Microsoft Docs website on the link below

Enable flow logging using Azure ExpressRoute Traffic Collector (Preview) | Microsoft Docs