Metasrv administration user guide

[killme2008]

Create an administration document for Metasrv, a critical database component storing table metadata. This document should include:

• Backend selection guide: Etcd vs. PostgreSQL, with configuration instructions.
• Metadata backup and restore procedures.
• Failover guidelines.
• REST API documentation for managing the Metasrv cluster and updating table metadata.
• Best practices
• Monitoring
• Troubleshooting
• Configurations, moved from https://docs.greptime.com/user-guide/deployments/configuration/#metasrv-only-configuration