Add insert rate, persistence rate, lines of code definitions (#1007)

Add definitions for the three new metrics we [recently
added](https://sourcegraph.slack.com/archives/C06358CMSRM/p1739491550008149)to
Sourcegraph Analytics (insert rate, persistence rate, lines of code)

Author: kelsey-brown

docs/analytics/index.mdx

@@ -73,6 +73,7 @@ Many of the metrics above are also available for Cody only. However, some user d
 | Completions by day                         | The number of completions suggested by day and by editor.                                                                                                                                                             |
 | Completion acceptance rate (CAR)           | The percent of completions presented to a user for at least 750ms accepted by day, the editor, day, and month.                                                                                                        |
 | Weighted completion acceptance rate (wCAR) | Similar to CAR, but weighted by the number of characters presented in the completion, by the editor, day, and month. This assigns more "weight" to accepted completions that provide more code to the user. |
+| Completion persistence rate | Percent of completions that are retained or mostly retained (67%+ of inserted text) after various time intervals. |
 | Average completion latency (ms)            | The average milliseconds of latency before a user is presented with a completion suggestion by an editor.                                                                                                             |
 | Acceptance rate by language                | CAR and total completion suggestions broken down by editor during the selected time                                                                                                                                   |
 
@@ -85,8 +86,11 @@ Many of the metrics above are also available for Cody only. However, some user d
 | Hours saved by chats                                 | Total hours saved through Cody chat interactions during the selected time , assuming 5 minutes saved per chat |
 | Cody chats by day                                    | Daily count of chat interactions                                                                              |
 | Cody chat users                                      | Daily count of chat users                                                                                     |
-| Prompts created, edited, and deleted by day          | Daily count of prompt management activities, including creation, modification, and removal                    |
-| Users creating, editing, and deleting prompts by day | Number of unique users performing prompt management activities each day                                       |
+| Lines of code inserted | Lines of code generated by Cody in chat that get applied, inserted, or pasted into the editor. Only VS Code is included in this metric for now |
+| Insert rate | Percent of code generated by Cody in chat that gets applied, inserted, or pasted into the editor. Only VS Code is included in this metric for now |
+| Chat apply & insert persistence rate | Percent of code inserted by Apply and Insert actions that are retained or mostly retained (67%+ of inserted text) after various time intervals |
+| Prompts created, edited, and deleted by day | Daily count of prompt management activities, including creation, modification, and removal |
+| Users creating, editing, and deleting prompts by day | Number of unique users performing prompt management activities each day |
 
 ### Command metrics (deprecated)