Add --json-log flag to enable structured JSON logging (#19968)

Co-authored-by: github_username <github_email>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sgl-model-gateway/src/main.rs

@@ -260,6 +260,10 @@ struct CliArgs {
     #[arg(long, default_value = "info", value_parser = ["debug", "info", "warn", "error"], help_heading = "Logging")]
     log_level: String,
 
+    /// Enable structured JSON log output instead of plain text
+    #[arg(long, default_value_t = false, help_heading = "Logging")]
+    json_log: bool,
+
     // ==================== Prometheus Metrics ====================
     /// Port to expose Prometheus metrics
     #[arg(long, default_value_t = 29000, help_heading = "Prometheus Metrics")]
@@ -1119,6 +1123,7 @@ impl CliArgs {
             max_payload_size: self.max_payload_size,
             log_dir: self.log_dir.clone(),
             log_level: Some(self.log_level.clone()),
+            json_log: self.json_log,
             service_discovery_config,
             prometheus_config,
             request_timeout_secs: self.request_timeout_secs,

sgl-model-gateway/src/server.rs

@@ -533,6 +533,7 @@ pub struct ServerConfig {
     pub max_payload_size: usize,
     pub log_dir: Option<String>,
     pub log_level: Option<String>,
+    pub json_log: bool,
     pub service_discovery_config: Option<ServiceDiscoveryConfig>,
     pub prometheus_config: Option<PrometheusConfig>,
     pub request_timeout_secs: u64,
@@ -722,7 +723,7 @@ pub async fn startup(config: ServerConfig) -> Result<(), Box<dyn std::error::Err
                         }
                     })
                     .unwrap_or(Level::INFO),
-                json_format: false,
+                json_format: config.json_log,
                 log_dir: config.log_dir.clone(),
                 colorize: true,
                 log_file_name: "smg".to_string(),