feat(vertexai): add release note for vertex ai apm tracing (#11548)

Add missing release note for this
[PR](#11236).

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

releasenotes/notes/feat-vertexai-e4a6dbe95d19eee0.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    vertexai: Introduces tracing support for Google's Vertex AI SDK for Python's ``generate_content`` and ``send_message`` calls.
+      See `the docs <https://ddtrace.readthedocs.io/en/stable/integrations.html#vertexai>`_
+      for more information.