feat: Implement Prometheus metrics exposure in SmartMetrics

- Added Prometheus gauges for CPU and memory metrics.
- Implemented HTTP server to expose metrics at /metrics endpoint.
- Created methods to enable and disable the Prometheus endpoint.
- Updated getMetrics() to set gauge values.
- Added tests for Prometheus metrics functionality.
- Updated documentation plan for Prometheus integration.

ts/smartmetrics.classes.smartmetrics.ts

@@ -7,11 +7,39 @@ export class SmartMetrics {
   public logger: plugins.smartlog.Smartlog;
   public registry: plugins.promClient.Registry;
   public maxMemoryMB: number;
+  
+  // Prometheus gauges for custom metrics
+  private cpuPercentageGauge: plugins.promClient.Gauge<string>;
+  private memoryPercentageGauge: plugins.promClient.Gauge<string>;
+  private memoryUsageBytesGauge: plugins.promClient.Gauge<string>;
+  
+  // HTTP server for Prometheus endpoint
+  private prometheusServer?: plugins.http.Server;
+  private prometheusPort?: number;
 
   public async setup() {
     const collectDefaultMetrics = plugins.promClient.collectDefaultMetrics;
     this.registry = new plugins.promClient.Registry();
     collectDefaultMetrics({ register: this.registry });
+    
+    // Initialize custom gauges
+    this.cpuPercentageGauge = new plugins.promClient.Gauge({
+      name: 'smartmetrics_cpu_percentage',
+      help: 'Current CPU usage percentage',
+      registers: [this.registry]
+    });
+    
+    this.memoryPercentageGauge = new plugins.promClient.Gauge({
+      name: 'smartmetrics_memory_percentage',
+      help: 'Current memory usage percentage',
+      registers: [this.registry]
+    });
+    
+    this.memoryUsageBytesGauge = new plugins.promClient.Gauge({
+      name: 'smartmetrics_memory_usage_bytes',
+      help: 'Current memory usage in bytes',
+      registers: [this.registry]
+    });
   }
 
   constructor(loggerArg: plugins.smartlog.Smartlog, sourceNameArg: string) {
@@ -100,6 +128,17 @@ export class SmartMetrics {
     )} / ${this.formatBytes(this.maxMemoryMB * 1024 * 1024)}`;
 
     console.log(`${cpuUsageText} ||| ${memoryUsageText} `);
+    
+    // Update Prometheus gauges with current values
+    if (this.cpuPercentageGauge) {
+      this.cpuPercentageGauge.set(cpuPercentage);
+    }
+    if (this.memoryPercentageGauge) {
+      this.memoryPercentageGauge.set(memoryPercentage);
+    }
+    if (this.memoryUsageBytesGauge) {
+      this.memoryUsageBytesGauge.set(memoryUsageBytes);
+    }
 
     const returnMetrics: interfaces.IMetricsSnapshot = {
       process_cpu_seconds_total: (
@@ -127,7 +166,58 @@ export class SmartMetrics {
     return returnMetrics;
   }
 
+  public async getPrometheusFormattedMetrics(): Promise<string> {
+    // Update metrics to ensure gauges have latest values
+    await this.getMetrics();
+    
+    // Return Prometheus text exposition format
+    return await this.registry.metrics();
+  }
+
+  public enablePrometheusEndpoint(port: number = 9090): void {
+    if (this.prometheusServer) {
+      this.logger.log('warn', 'Prometheus endpoint is already running');
+      return;
+    }
+    
+    this.prometheusServer = plugins.http.createServer(async (req, res) => {
+      if (req.url === '/metrics' && req.method === 'GET') {
+        try {
+          const metrics = await this.getPrometheusFormattedMetrics();
+          res.writeHead(200, { 'Content-Type': 'text/plain; version=0.0.4' });
+          res.end(metrics);
+        } catch (error) {
+          res.writeHead(500, { 'Content-Type': 'text/plain' });
+          res.end('Error generating metrics');
+          this.logger.log('error', 'Error generating Prometheus metrics', error);
+        }
+      } else {
+        res.writeHead(404, { 'Content-Type': 'text/plain' });
+        res.end('Not Found');
+      }
+    });
+    
+    this.prometheusPort = port;
+    this.prometheusServer.listen(port, () => {
+      this.logger.log('info', `Prometheus metrics endpoint available at http://localhost:${port}/metrics`);
+    });
+  }
+
+  public disablePrometheusEndpoint(): void {
+    if (!this.prometheusServer) {
+      return;
+    }
+    
+    this.prometheusServer.close(() => {
+      this.logger.log('info', `Prometheus metrics endpoint on port ${this.prometheusPort} has been shut down`);
+    });
+    
+    this.prometheusServer = undefined;
+    this.prometheusPort = undefined;
+  }
+
   public stop() {
     this.started = false;
+    this.disablePrometheusEndpoint();
   }
 }