Add OpenTelemetry instrumentation with middleware-only approach

config.example.yaml

@@ -22,9 +22,40 @@ logging:
   # When true, logs are output in JSON format instead of console format
   json: false
 
+# Telemetry configuration (OpenTelemetry)
+telemetry:
+  # Enable OpenTelemetry tracing (default: false)
+  enabled: false
+  
+  # OTLP endpoint for trace export (default: "localhost:4317")
+  # Format: host:port
+  otlp_endpoint: "localhost:4317"
+  
+  # Service name for tracing (default: "DanceLessonsCoach")
+  service_name: "DanceLessonsCoach"
+  
+  # Use insecure connection (no TLS) (default: true)
+  insecure: true
+  
+  # Sampler configuration
+  sampler:
+    # Sampler type (default: "parentbased_always_on")
+    # Options: "always_on", "always_off", "traceidratio", "parentbased_always_on", "parentbased_always_off", "parentbased_traceidratio"
+    type: "parentbased_always_on"
+    
+    # Sampling ratio (0.0 to 1.0, default: 1.0)
+    # Only used with traceidratio and parentbased_traceidratio samplers
+    ratio: 1.0
+
 # Environment Variables
 # You can also configure via environment variables with DLC_ prefix:
 # DLC_SERVER_HOST=0.0.0.0
 # DLC_SERVER_PORT=8080
 # DLC_SHUTDOWN_TIMEOUT=30s
-# DLC_LOGGING_JSON=false
+# DLC_LOGGING_JSON=false
+# DLC_TELEMETRY_ENABLED=true
+# DLC_TELEMETRY_OTLP_ENDPOINT="jaeger:4317"
+# DLC_TELEMETRY_SERVICE_NAME="DanceLessonsCoach"
+# DLC_TELEMETRY_INSECURE=true
+# DLC_TELEMETRY_SAMPLER_TYPE="parentbased_always_on"
+# DLC_TELEMETRY_SAMPLER_RATIO=1.0