Remove all unused code and fix build warnings

- Remove unused struct fields: tier, config_name, last_collection_time
- Remove unused structs: PerformanceMetrics, PerfMonitor
- Remove unused methods: get_performance_metrics, get_collector_names, get_stats
- Remove unused utility functions and system helpers
- Remove unused config fields from CPU and Memory collectors
- Keep config fields that are actually used (DiskCollector, etc.)
- Remove unused proxy_pass_url variable and assignments
- Fix duplicate hostname variable declaration
- Achieve zero build warnings without functionality changes

agent/src/metrics/mod.rs

@@ -237,21 +237,6 @@ impl MetricCollectionManager {
         Ok(all_metrics)
     }
 
-    /// Get names of all registered collectors
-    pub fn get_collector_names(&self) -> Vec<String> {
-        self.collectors
-            .iter()
-            .map(|c| c.name().to_string())
-            .collect()
-    }
-
-    /// Get collector statistics
-    pub fn get_stats(&self) -> HashMap<String, bool> {
-        self.collectors
-            .iter()
-            .map(|c| (c.name().to_string(), true)) // All collectors are enabled
-            .collect()
-    }
 
     /// Get all cached metrics from the cache manager
     pub async fn get_all_cached_metrics(&self) -> Result<Vec<Metric>> {
@@ -263,20 +248,4 @@ impl MetricCollectionManager {
         Ok(cached_metrics)
     }
 
-    /// Determine which collector handles a specific metric
-    fn get_collector_for_metric(&self, metric_name: &str) -> String {
-        if metric_name.starts_with("cpu_") {
-            "cpu".to_string()
-        } else if metric_name.starts_with("memory_") {
-            "memory".to_string()
-        } else if metric_name.starts_with("disk_") {
-            "disk".to_string()
-        } else if metric_name.starts_with("service_") {
-            "systemd".to_string()
-        } else if metric_name.starts_with("backup_") {
-            "backup".to_string()
-        } else {
-            "unknown".to_string()
-        }
-    }
 }