Support TiDB Cloud Next-Gen

[djshow832]

Feature Request

Design

• RFC

Make it run

• Fetch TiDB info of all keyspaces from etcd #839
• Support routing based on CIDR #845
• Pass the client IP in the routing context #849
• Capture/replay traffic of one keyspace
• Write metering data to S3 #857
• Support updating CIDR dynamically #932
• Meter public and private traffic separately #990

Improve scalability

• Optimize query metrics from backends
• Optimize health check