lua: add stats (#22623)

Currently stats are not available in lua filter, which makes it hard to
track/alert for the script errors. This change adds error stat.

Signed-off-by: Suresh Kumar <sureshkumar.pp@gmail.com>
Signed-off-by: Suresh Kumar <suresh.ponnusamy@freshworks.com>

Mirrored from https://github.com/envoyproxy/envoy @ 8c88943b747bd74517eaf257cd75dd686aa7bbb8

envoy/extensions/filters/http/lua/v3/lua.proto

@@ -58,6 +58,26 @@ message Lua {
   // The default Lua code that Envoy will execute. If no per route config is provided
   // for the request, this Lua code will be applied.
   config.core.v3.DataSource default_source_code = 3;
+
+  // Optional additional prefix to use when emitting statistics. By default
+  // metrics are emitted in *.lua.* namespace. If multiple lua filters are
+  // configured in a filter chain, the stats from each filter instance can
+  // be emitted using custom stat prefix to distinguish emitted
+  // statistics. For example:
+  //
+  // .. code-block:: yaml
+  //
+  //   http_filters:
+  //     - name: envoy.filters.http.lua
+  //       typed_config:
+  //         "@type": type.googleapis.com/envoy.extensions.filters.http.lua.v3.Lua
+  //         stat_prefix: foo_script # This emits lua.foo_script.errors etc.
+  //     - name: envoy.filters.http.lua
+  //       typed_config:
+  //         "@type": type.googleapis.com/envoy.extensions.filters.http.lua.v3.Lua
+  //         stat_prefix: bar_script # This emits lua.bar_script.errors etc.
+  //
+  string stat_prefix = 4;
 }
 
 message LuaPerRoute {