feat(plan): create a metric for execution of plans generated in plan mode (#18236)

docs/cli/telemetry.md

@@ -320,6 +320,8 @@ Captures startup configuration and user prompt submissions.
 
 Tracks changes and duration of approval modes.
 
+##### Lifecycle
+
 - `approval_mode_switch`: Approval mode was changed.
   - **Attributes**:
     - `from_mode` (string)
@@ -330,6 +332,15 @@ Tracks changes and duration of approval modes.
     - `mode` (string)
     - `duration_ms` (int)
 
+##### Execution
+
+These events track the execution of an approval mode, such as Plan Mode.
+
+- `plan_execution`: A plan was executed and the session switched from plan mode
+  to active execution.
+  - **Attributes**:
+    - `approval_mode` (string)
+
 #### Tools
 
 Captures tool executions, output truncation, and Edit behavior.
@@ -710,6 +721,17 @@ Agent lifecycle metrics: runs, durations, and turns.
   - **Attributes**:
     - `agent_name` (string)
 
+##### Approval Mode
+
+###### Execution
+
+These metrics track the adoption and usage of specific approval workflows, such
+as Plan Mode.
+
+- `gemini_cli.plan.execution.count` (Counter, Int): Counts plan executions.
+  - **Attributes**:
+    - `approval_mode` (string)
+
 ##### UI
 
 UI stability signals such as flicker count.