Add capability to parse forward headers

In case standalone watcher-api runs behind forwarders (like load
balancers), it should parse specific request headers to determine
the endpoint url clients actually use.

Add http_proxy_to_wsgi middleware to api pipeline to handle this.

Closes-Bug: #2122353
Change-Id: I27ade17f7ce1649295f92f3ea1af620df63ba1bc
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

etc/watcher/oslo-config-generator/watcher.conf

@@ -9,6 +9,7 @@ namespace = oslo.concurrency
 namespace = oslo.db
 namespace = oslo.log
 namespace = oslo.messaging
+namespace = oslo.middleware.http_proxy_to_wsgi
 namespace = oslo.policy
 namespace = oslo.reports
 namespace = oslo.service.periodic_task

releasenotes/notes/http_proxy_to_wsgi-81ab98b39038fb39.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    The ``http_proxy_to_wsgi`` middleware has been added to the api pipeline.
+    Now setting the ``[oslo_middleware] enable_proxy_headers_parsing`` option
+    to true enables parsing the HTTP headers set by forwarders, to detect
+    endpoint urls clients actually use.
+

watcher/api/app.py

@@ -17,6 +17,7 @@
 #    under the License.
 
 
+from oslo_middleware import http_proxy_to_wsgi
 from oslo_middleware import request_id
 import pecan
 
@@ -56,6 +57,8 @@ def _wrap_app(app):
 
     app = request_id.RequestId(app)
 
+    app = http_proxy_to_wsgi.HTTPProxyToWSGI(app)
+
     return app