Run nova-api and -metadata in threaded mode

It turns out that nova-api and nova-metadata only depend on spawning
threads via scatter-gather. The scatter-gather already supports both
eventlet and threading mode so we can switch these services.

Our WSGI services (nova-api, nova-metadata) are not relying on
oslo.service to fork worker processes, but expect the web server to
handle that (uwsgi, apache mod_wsgi). This means we don't need to handle
any forking issues as no nova code runs before the fork.

Change-Id: Id3a339c605dfc730bdb7994c3ca45baafeb5af80
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

.zuul.yaml

@@ -475,6 +475,8 @@
         # https://bugs.launchpad.net/nova/+bug/2110545
         NOVA_COMPILE_MDEV_SAMPLES: False
         'SYSTEMD_ENV_VARS["n-sch"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
+        'SYSTEMD_ENV_VARS["n-api"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
+        'SYSTEMD_ENV_VARS["n-api-meta"]': OS_NOVA_DISABLE_EVENTLET_PATCHING=true
       devstack_services:
         # Disable OVN services
         br-ex-tcpdump: false

doc/source/admin/concurrency.rst

@@ -24,8 +24,8 @@ threading mode at service startup via setting the environment variable
 
 .. note::
 
-   Since nova 32.0.0 (2025.2 Flamingo) the nova-scheduler can be switched to
-   native threading mode.
+   Since nova 32.0.0 (2025.2 Flamingo) the nova-scheduler, nova-metadata, and
+   nova-api can be switched to native threading mode.
 
 
 Tunables for the native threading mode

releasenotes/notes/threaded-nova-api-d8ec6704dcf66fb3.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    The nova-api and nova-metadata services now can be run in native threading
+    mode instead of with eventlet. This is an experimental feature that is
+    disabled by default. Please read the
+    `concurrency <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
+    guide for more details.