Adding notes for ext_proc runtime flag (envoyproxy#41690)

Add a do-not-deprecate runtime flag notes

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Author: yanjunxiang-google

source/common/runtime/runtime_features.cc

@@ -41,6 +41,8 @@ RUNTIME_GUARD(envoy_reloadable_features_disallow_quic_client_udp_mmsg);
 RUNTIME_GUARD(envoy_reloadable_features_enable_cel_regex_precompilation);
 RUNTIME_GUARD(envoy_reloadable_features_enable_compression_bomb_protection);
 RUNTIME_GUARD(envoy_reloadable_features_enable_new_query_param_present_match_behavior);
+// Ignore the automated "remove this flag" issue: we should keep this for 1 year. Confirm with
+// @yanjunxiang-google before removing.
 RUNTIME_GUARD(envoy_reloadable_features_ext_proc_fail_close_spurious_resp);
 RUNTIME_GUARD(envoy_reloadable_features_generic_proxy_codec_buffer_limit);
 RUNTIME_GUARD(envoy_reloadable_features_grpc_side_stream_flow_control);