AWS Transit Gateway and AWS Cloud WAN enhance visibility metrics and Path MTU support

AWS Transit Gateway (TGW) and AWS Cloud WAN now support per availability zone (AZ) metrics delivered to CloudWatch. Furthermore, both services now support Path Maximum Transmission Unit Discovery (PMTUD) for effective mitigation against MTU mismatch issues in their global networks. TGW and Cloud WAN allow customers to monitor their global network through performance and traffic metrics such as bytes in/out, packets in/out, and packets dropped. Until now, these metrics were available at an attachment level, and aggregate TGW and Core Network Edge (CNE) levels. With this launch, customers have more granular visibility into AZ-level metrics for VPC attachments. AZ-level metrics enable customers to rapidly troubleshoot any AZ impairments and provide deeper visibility in AZ-level traffic patterns across TGW and Cloud WAN. TGW and Cloud WAN now also support standard PMTUD mechanism for traffic ingressing on VPC attachments. Until now, jumbo sized packets exceeding the TGW/CNE MTU (8500 bytes) would get silently dropped on VPC attachments. With this launch, an Internet Control Message Protocol (ICMP) _Fragmentation Needed_ response message is sent back to sender hosts allowing them to remediate packet MTU size and thus minimize packet loss due to MTU mismatches in their network. PMTUD support is available for both IPv4 and IPv6 packets. The per-AZ CloudWatch metrics and PMTUD support are available within each service in all AWS Regions where TGW or Cloud WAN are available. For more information, see the [AWS Transit Gateway](https://docs.aws.amazon.com/vpc/latest/tgw/transit-gateway-cloudwatch-metrics.html) and [AWS Cloud WAN](https://docs.aws.amazon.com/network-manager/latest/cloudwan/cloudwan-metrics.html) documentation pages.