[stackdriver addon] Bump prometheus-to-sd to v0.5.0 to pick up security fixes.
[fluentd-gcp addon] Bump fluentd-gcp-scaler to v0.5.1 to pick up security fixes.
[fluentd-gcp addon] Bump event-exporter to v0.2.4 to pick up security fixes.
[fluentd-gcp addon] Bump prometheus-to-sd to v0.5.0 to pick up security fixes.
[metatada-proxy addon] Bump prometheus-to-sd v0.5.0 to pick up security fixes.