Set Content-Length for Metrics requests (#7730)

HTTP requires the response to contain a Content-Length header unless chunked encoding is being used.
Prometheus metrics endpoint did not set this, causing software such as prometheus-proxy to not be able to scrape synapse for metrics.

Signed-off-by: Christian Svensson <blue@cmd.nu>

changelog.d/7730.bugfix

@@ -0,0 +1 @@
+Fix missing `Content-Length` on HTTP responses from the metrics handler.

synapse/metrics/_exposition.py

@@ -208,6 +208,7 @@ class MetricsHandler(BaseHTTPRequestHandler):
             raise
         self.send_response(200)
         self.send_header("Content-Type", CONTENT_TYPE_LATEST)
+        self.send_header("Content-Length", str(len(output)))
         self.end_headers()
         self.wfile.write(output)
 
@@ -261,4 +262,6 @@ class MetricsResource(Resource):
 
     def render_GET(self, request):
         request.setHeader(b"Content-Type", CONTENT_TYPE_LATEST.encode("ascii"))
-        return generate_latest(self.registry)
+        response = generate_latest(self.registry)
+        request.setHeader(b"Content-Length", str(len(response)))
+        return response