add session.change event

.chloggen/session_change_event.yaml

@@ -0,0 +1,22 @@
+# Use this changelog template to create an entry for release notes.
+#
+# If your change doesn't affect end users you should instead start
+# your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: 'enhancement'
+
+# The name of the area of concern in the attributes-registry, (e.g. http, cloud, db)
+component: 'session'
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add new experimental `session.change` event
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+# The values here must be integers.
+issues: []
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

docs/general/session.md

@@ -13,7 +13,7 @@ the Logs, Events, and Spans generated during the Session's lifecycle.
 
 When a session reaches end of life, typically due to user inactivity or session timeout, a new session identifier
 will be assigned. The previous session identifier may be provided by the instrumentation so that telemetry
-backends can link the two sessions.
+backends can link the two sessions (see [Session Change Event](#session_change_event) below).
 
 ## Attributes
 
@@ -35,4 +35,14 @@ backends can link the two sessions.
 <!-- END AUTOGENERATED TEXT -->
 <!-- endsemconv -->
 
+## Session Change Event
+
+![Experimental](https://img.shields.io/badge/-experimental-blue)
+
+When a session changes (due to time-based expiry or another mechanism), an event MAY be emitted. 
+
+The emitted event that represents a session change MUST have the `event.name=session.change`.
+The event body MUST be empty and the attributes MUST include both the `session.id` and `session.previous_id`
+attributes, as described above. The values MUST be different.
+
 [DocumentStatus]: https://github.com/open-telemetry/opentelemetry-specification/tree/v1.31.0/specification/document-status.md