Modern distributed systems are complex. A single user request might travel through multiple services, databases, caches, queues, and external APIs before a response is returned. User │ ▼ API Gateway │ ▼ User Service ───→ Redis │ ▼ Order Service │ ▼ Payment Service │ ▼ Database Now imagine a user reports: "The application is very slow." Where is the problem? Is it: The API server? The database? Redis? A slow microservice? An ext...