fix(prometheus): increase proxy buffer size for oauth2

Increase proxy buffer size for OAuth2 Proxy so that bigger response
headers are handled correctly.

Author: d3adb5

argo/app-of-apps/templates/monitoring/kube-prometheus-stack.yaml

@@ -34,6 +34,7 @@ spec:
             className: nginx
             annotations:
               cert-manager.io/cluster-issuer: letsencrypt
+              nginx.ingress.kubernetes.io/proxy-buffer-size: "8k"
             hosts:
               - "prometheus.d3adb5.ca"
             path: /oauth2