Document PROXY

Author: jorbaum

deploy-apps/streaming-logs.html.md.erb

@@ -157,6 +157,10 @@ For example:
 
 The Diego Cell emits `HEALTH` logs when running health check actions for an app. These logs are generated by the health check binary that monitors app instances. By default, health check log output is suppressed unless an error occurs.
 
+### <a id='proxy'></a> PROXY
+
+The Envoy sidecar proxy emits `PROXY` logs. Each app container includes an Envoy sidecar that handles the mTLS handshake with the Gorouter and proxies traffic to the app process. By default, Envoy runs with a critical log level, so `PROXY` logs mainly appear when Envoy encounters critical errors or fails to start.
+
 
 ## <a id='writing'></a> Writing to the log from your app