add --telemetry-outfile flag (#4689)

docs/telemetry.md

@@ -19,6 +19,7 @@ The following lists the precedence for applying telemetry settings, with items l
     - `--telemetry-target <local|gcp>`: Overrides `telemetry.target`.
     - `--telemetry-otlp-endpoint <URL>`: Overrides `telemetry.otlpEndpoint`.
     - `--telemetry-log-prompts` / `--no-telemetry-log-prompts`: Overrides `telemetry.logPrompts`.
+    - `--telemetry-outfile <path>`: Redirects telemetry output to a file. See [Exporting to a file](#exporting-to-a-file).
 
 1.  **Environment variables:**
     - `OTEL_EXPORTER_OTLP_ENDPOINT`: Overrides `telemetry.otlpEndpoint`.
@@ -50,6 +51,16 @@ The following code can be added to your workspace (`.gemini/settings.json`) or u
 }
 ```
 
+### Exporting to a file
+
+You can export all telemetry data to a file for local inspection.
+
+To enable file export, use the `--telemetry-outfile` flag with a path to your desired output file. This must be run using `--telemetry-target=local`.
+
+```bash
+gemini --telemetry --telemetry-target=local --telemetry-outfile=/path/to/telemetry.log "your prompt"
+```
+
 ## Running an OTEL Collector
 
 An OTEL Collector is a service that receives, processes, and exports telemetry data.