add some prometheus metrics

Author: Lukas Wingerberg

server/grpc-ffmpeg.py

@@ -220,7 +220,8 @@ async def health_check(request):
             return web.Response(text="Health check failed", status=500)
 
     async def metrics(request):
-        return web.Response(body=generate_latest(), content_type=CONTENT_TYPE_LATEST)
+
+        return web.Response(body=generate_latest(), headers={"Content-Type": CONTENT_TYPE_LATEST})
 
     app = web.Application()
     app.router.add_get('/health', health_check)