ZooKeeper proxy filter (#5991)

This filter decodes the ZooKeeper wire protocol and emits
stats & metadata about requests, responses and events.

This wire protocol parsing is based on:

https://github.com/twitter/zktraffic
https://github.com/rgs1/zktraffic-cpp

The actual filter structure is based on the Mysql proxy filter.

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Mirrored from https://github.com/envoyproxy/envoy @ b771f99f53142bbb1496dbcddccc5defaaaa8dd8

envoy/config/filter/network/zookeeper_proxy/v1alpha1/BUILD

@@ -0,0 +1,8 @@
+load("//bazel:api_build_system.bzl", "api_proto_library_internal")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_library_internal(
+    name = "zookeeper_proxy",
+    srcs = ["zookeeper_proxy.proto"],
+)

envoy/config/filter/network/zookeeper_proxy/v1alpha1/zookeeper_proxy.proto

@@ -0,0 +1,33 @@
+syntax = "proto3";
+
+package envoy.config.filter.network.zookeeper_proxy.v1alpha1;
+
+option java_outer_classname = "ZookeeperProxyProto";
+option java_multiple_files = true;
+option java_package = "io.envoyproxy.envoy.config.filter.network.zookeeper_proxy.v1alpha1";
+option go_package = "v1alpha1";
+
+import "validate/validate.proto";
+import "google/protobuf/wrappers.proto";
+
+// [#protodoc-title: ZooKeeper proxy]
+// ZooKeeper Proxy :ref:`configuration overview <config_network_filters_zookeeper_proxy>`.
+message ZooKeeperProxy {
+  // The human readable prefix to use when emitting :ref:`statistics
+  // <config_network_filters_zookeeper_proxy_stats>`.
+  string stat_prefix = 1 [(validate.rules).string.min_bytes = 1];
+
+  // [#not-implemented-hide:] The optional path to use for writing ZooKeeper access logs.
+  // If the access log field is empty, access logs will not be written.
+  string access_log = 2;
+
+  // Messages — requests, responses and events — that are bigger than this value will
+  // be ignored. If it is not set, the default value is 1Mb.
+  //
+  // The value here should match the jute.maxbuffer property in your cluster configuration:
+  //
+  // https://zookeeper.apache.org/doc/r3.4.10/zookeeperAdmin.html#Unsafe+Options
+  //
+  // if that is set. If it isn't, ZooKeeper's default is also 1Mb.
+  google.protobuf.UInt32Value max_packet_bytes = 3;
+}