messaging: add messaging_latest_experimental opt-in value

Author: pichlermarc

.chloggen/feat_experimental_messaging.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: "messaging"
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Add messaging_latest_experimental feature flag to enable experimental messaging conventions"
+
+# 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: [2928]
+
+# (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/messaging/README.md

@@ -24,12 +24,17 @@ This document defines semantic conventions for messaging systems spans, metrics
 >   * `messaging` - emit the new, stable messaging conventions,
 >     and stop emitting the old experimental messaging conventions
 >     that the instrumentation emitted previously.
+>   * `messaging_latest_experimental` - emit the latest experimental version of
+      Messaging semantic conventions (supported by the instrumentation) and do not emit the
+      old one (v1.24.0 or prior).
 >   * `messaging/dup` - emit both the old and the stable messaging conventions,
 >     allowing for a seamless transition.
 >   * The default behavior (in the absence of one of these values) is to continue
 >     emitting whatever version of the old experimental messaging conventions
 >     the instrumentation was emitting previously.
->   * Note: `messaging/dup` has higher precedence than `messaging` in case both values are present
+>   * Notes: 
+>     * `messaging/dup` has higher precedence than `messaging` in case both values are present
+>     * `messaging_latest_experimental` has higher precedence than both `messaging/dup` and `messaging`
 > * SHOULD maintain (security patching at a minimum) the existing major version
 >   for at least six months after it starts emitting both sets of conventions.
 > * SHOULD drop the environment variable in the next major version.