Add connection close telemetry event

Implement CONNECTION_CLOSE telemetry event to track session lifecycle:
- Add CONNECTION_CLOSE event type to TelemetryEventType enum
- Add emitConnectionClose() method to TelemetryEventEmitter
- Add processConnectionCloseEvent() handler in MetricsAggregator
- Track session open time in DBSQLSession and emit close event with latency
- Remove unused TOperationType import from DBSQLOperation

This provides complete session telemetry: connection open, statement execution,
and connection close with latencies for each operation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: samikshya-db

README.md

@@ -84,15 +84,15 @@ To enable or disable telemetry explicitly:
 
 ```javascript
 const client = new DBSQLClient({
-  telemetryEnabled: true,  // Enable telemetry (default: false)
+  telemetryEnabled: true, // Enable telemetry (default: false)
 });
 
 // Or override per connection:
 await client.connect({
   host: '********.databricks.com',
   path: '/sql/2.0/warehouses/****************',
   token: 'dapi********************************',
-  telemetryEnabled: false,  // Disable for this connection
+  telemetryEnabled: false, // Disable for this connection
 });
 ```
 

docs/TELEMETRY.md

@@ -13,7 +13,6 @@ import IOperation, {
 import {
   TGetOperationStatusResp,
   TOperationHandle,
-  TOperationType,
   TTableSchema,
   TSparkDirectResults,
   TGetResultSetMetadataResp,
@@ -509,7 +508,7 @@ export default class DBSQLOperation implements IOperation {
    */
   private emitStatementStart(): void {
     try {
-      const {telemetryEmitter} = (this.context as any);
+      const { telemetryEmitter } = this.context as any;
       if (!telemetryEmitter) {
         return;
       }
@@ -530,8 +529,8 @@ export default class DBSQLOperation implements IOperation {
    */
   private async emitStatementComplete(): Promise<void> {
     try {
-      const {telemetryEmitter} = (this.context as any);
-      const {telemetryAggregator} = (this.context as any);
+      const { telemetryEmitter } = this.context as any;
+      const { telemetryAggregator } = this.context as any;
       if (!telemetryEmitter || !telemetryAggregator) {
         return;
       }
@@ -571,7 +570,7 @@ export default class DBSQLOperation implements IOperation {
    */
   private emitErrorEvent(error: Error): void {
     try {
-      const {telemetryEmitter} = (this.context as any);
+      const { telemetryEmitter } = this.context as any;
       if (!telemetryEmitter) {
         return;
       }

lib/DBSQLOperation.ts

@@ -151,6 +151,8 @@ export default class DBSQLSession implements IDBSQLSession {
 
   private isOpen = true;
 
+  private openTime: number;
+
   private serverProtocolVersion?: TProtocolVersion;
 
   public onClose?: () => void;
@@ -169,6 +171,7 @@ export default class DBSQLSession implements IDBSQLSession {
   constructor({ handle, context, serverProtocolVersion }: DBSQLSessionConstructorOptions) {
     this.sessionHandle = handle;
     this.context = context;
+    this.openTime = Date.now();
     // Get the server protocol version from the provided parameter (from TOpenSessionResp)
     this.serverProtocolVersion = serverProtocolVersion;
     this.context.getLogger().log(LogLevel.debug, `Session created with id: ${this.id}`);
@@ -594,6 +597,16 @@ export default class DBSQLSession implements IDBSQLSession {
     this.onClose?.();
     this.isOpen = false;
 
+    // Emit connection close telemetry
+    const closeLatency = Date.now() - this.openTime;
+    const { telemetryEmitter } = this.context as any;
+    if (telemetryEmitter) {
+      telemetryEmitter.emitConnectionClose({
+        sessionId: this.id,
+        latencyMs: closeLatency,
+      });
+    }
+
     this.context.getLogger().log(LogLevel.debug, `Session closed with id: ${this.id}`);
     return new Status(response.status);
   }

lib/DBSQLSession.ts

@@ -145,7 +145,7 @@ export default class CloudFetchResultHandler implements IResultsProvider<ArrowBa
         return;
       }
 
-      const {telemetryEmitter} = (this.context as any);
+      const { telemetryEmitter } = this.context as any;
       if (!telemetryEmitter) {
         return;
       }

lib/result/CloudFetchResultHandler.ts

@@ -106,6 +106,11 @@ export default class MetricsAggregator {
         return;
       }
 
+      if (event.eventType === TelemetryEventType.CONNECTION_CLOSE) {
+        this.processConnectionCloseEvent(event);
+        return;
+      }
+
       // Error events - check if terminal or retryable
       if (event.eventType === TelemetryEventType.ERROR) {
         this.processErrorEvent(event);
@@ -143,6 +148,21 @@ export default class MetricsAggregator {
     this.addPendingMetric(metric);
   }
 
+  /**
+   * Process connection close event (emit immediately)
+   */
+  private processConnectionCloseEvent(event: TelemetryEvent): void {
+    const metric: TelemetryMetric = {
+      metricType: 'connection',
+      timestamp: event.timestamp,
+      sessionId: event.sessionId,
+      driverConfig: this.driverConfig,
+      latencyMs: event.latencyMs,
+    };
+
+    this.addPendingMetric(metric);
+  }
+
   /**
    * Process error event (terminal errors flushed immediately, retryable buffered)
    */

lib/telemetry/MetricsAggregator.ts

@@ -70,6 +70,29 @@ export default class TelemetryEventEmitter extends EventEmitter {
     }
   }
 
+  /**
+   * Emit a connection close event.
+   *
+   * @param data Connection close event data including sessionId and latencyMs
+   */
+  emitConnectionClose(data: { sessionId: string; latencyMs: number }): void {
+    if (!this.enabled) return;
+
+    const logger = this.context.getLogger();
+    try {
+      const event: TelemetryEvent = {
+        eventType: TelemetryEventType.CONNECTION_CLOSE,
+        timestamp: Date.now(),
+        sessionId: data.sessionId,
+        latencyMs: data.latencyMs,
+      };
+      this.emit(TelemetryEventType.CONNECTION_CLOSE, event);
+    } catch (error: any) {
+      // Swallow all exceptions - log at debug level only
+      logger.log(LogLevel.debug, `Error emitting connection close event: ${error.message}`);
+    }
+  }
+
   /**
    * Emit a statement start event.
    *

lib/telemetry/TelemetryEventEmitter.ts

@@ -24,6 +24,7 @@ export const DRIVER_NAME = 'nodejs-sql-driver';
  */
 export enum TelemetryEventType {
   CONNECTION_OPEN = 'connection.open',
+  CONNECTION_CLOSE = 'connection.close',
   STATEMENT_START = 'statement.start',
   STATEMENT_COMPLETE = 'statement.complete',
   CLOUDFETCH_CHUNK = 'cloudfetch.chunk',