feat(usage): add in-memory usage statistics tracking and API endpoint

- Introduced in-memory request statistics aggregation in `LoggerPlugin`.
- Added new structures for detailed metrics collection (e.g., token breakdown, request success/failure).
- Implemented `/usage` management API endpoint for retrieving aggregated statistics.
- Updated management handlers to support the new usage statistics functionality.
- Enhanced documentation to describe the usage metrics API.

internal/api/server.go

@@ -271,6 +271,7 @@ func (s *Server) setupRoutes() {
 		mgmt := s.engine.Group("/v0/management")
 		mgmt.Use(s.mgmt.Middleware())
 		{
+			mgmt.GET("/usage", s.mgmt.GetUsageStatistics)
 			mgmt.GET("/config", s.mgmt.GetConfig)
 
 			mgmt.GET("/debug", s.mgmt.GetDebug)