PrometheusProxy: Add support for authorization

Now that our Shard model supports an "authorization" field to specify the value
of the HTTP "Authorization" header, we can use it in our PrometheusProxy class.

Note that we have replace "host" with "shard" since the objects obtained by the
'models.Shard.objects.filter(proxy=True)' instruction are shards, not hosts.

promgen/proxy.py

@@ -39,7 +39,7 @@ class PrometheusProxy(View):
     proxy_headers = {"HTTP_REFERER": "Referer"}
 
     @property
-    def headers(self):
+    def request_headers(self):
         # Loop through the headers from our request, and decide which ones
         # we should pass through upstream. Currently, our 'Referer' header is
         # the main one we are interested in, since this can help us debug which
@@ -53,12 +53,15 @@ def headers(self):
     def proxy(self, request):
         futures = []
         with concurrent.futures.ThreadPoolExecutor(max_workers=20) as executor:
-            for host in models.Shard.objects.filter(proxy=True):
+            for shard in models.Shard.objects.filter(proxy=True):
+                headers = self.request_headers
+                if shard.authorization:
+                    headers["Authorization"] = shard.authorization
                 futures.append(
                     executor.submit(
                         util.get,
-                        urljoin(host.url, request.get_full_path_info()),
-                        headers=self.headers,
+                        urljoin(shard.url, request.get_full_path_info()),
+                        headers=headers,
                     )
                 )
             yield from concurrent.futures.as_completed(futures)