docs: add chat_panel_plan_mode to Copilot usage metrics API and export fields (#61255)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kyanny <10515+kyanny@users.noreply.github.com>
Co-authored-by: mc <42146119+mchammer01@users.noreply.github.com>

Author: 3 people

content/copilot/reference/copilot-usage-metrics/copilot-usage-metrics.md

@@ -80,6 +80,7 @@ For example schemas of the data returned by the APIs, see [AUTOTITLE](/copilot/r
 | `chat_panel_ask_mode` | Captures user-initiated interactions in the chat panel with ask mode selected. |
 | `chat_panel_custom_mode` | Captures user-initiated interactions in the chat panel with a custom agent selected. |
 | `chat_panel_edit_mode` | Captures user-initiated interactions in the chat panel with edit mode selected. |
+| `chat_panel_plan_mode` | Captures user-initiated interactions in the chat panel with plan mode selected. |
 | `chat_panel_unknown_mode` | Captures user-initiated interactions in the chat panel where the mode is unknown. |
 | `code_generation_activity_count` | Number of distinct {% data variables.product.prodname_copilot_short %} output events generated. <br><br> **Includes:** All generated content, including comments and docstrings. <br> **Multiple blocks:** Each distinct code block from a single user prompt counts as a separate generation. <br> **Note:** This metric is not directly comparable to `user_initiated_interaction_count`, since one prompt can produce multiple generations. |
 | `code_acceptance_activity_count` | Number of suggestions or code blocks accepted by users. <br><br> **Counts:** All built-in accept actions, such as “apply to file,” “insert at cursor,” “insert into terminal,” and use of the **Copy** button. <br> **Does not count:** Manual OS clipboard actions (for example, <kbd>Ctrl</kbd>+<kbd>C</kbd>). <br> **Granularity:** Each acceptance action increments the count once, regardless of how many code blocks were generated by the initial prompt. |