Flip runtime guard on use_fast_protobuf_hash (#33121)

---------

Signed-off-by: Raven Black <ravenblack@dropbox.com>

changelogs/current.yaml

@@ -94,6 +94,12 @@ minor_behavior_changes:
  Disables recvmmsg (multi-message) for reading packets from a client QUIC UDP socket, if GRO
  is not set or not supported. recvmsg will be used instead. This behavior change can be
  reverted by setting ``envoy.reloadable_features.disallow_quic_client_udp_mmsg`` to ``false``.
+- area: config parsing, http cache filter
+ change: |
+ ``envoy.restart_features.use_fast_protobuf_hash`` was flipped to true by default.
+ The expectation is that this will improve performance of the hash operation by 2x to 10x,
+ and reduce config update time by 10-25%. This change will also cause a one-time cache flush
+ for file_system_http_cache.
 - area: xds
  change: |
  Delta SDS removals will no longer result in a "Missing SDS resources" NACK

source/common/runtime/runtime_features.cc

@@ -108,6 +108,7 @@ RUNTIME_GUARD(envoy_restart_features_allow_client_socket_creation_failure);
 RUNTIME_GUARD(envoy_restart_features_quic_handle_certs_with_shared_tls_code);
 RUNTIME_GUARD(envoy_restart_features_send_goaway_for_premature_rst_streams);
 RUNTIME_GUARD(envoy_restart_features_udp_read_normalize_addresses);
+RUNTIME_GUARD(envoy_restart_features_use_fast_protobuf_hash);
 
 // Begin false flags. Most of them should come with a TODO to flip true.
 
@@ -149,8 +150,6 @@ FALSE_RUNTIME_GUARD(envoy_restart_features_use_eds_cache_for_ads);
 FALSE_RUNTIME_GUARD(envoy_reloadable_features_enable_universal_header_validator);
 // TODO(pksohn): enable after fixing https://github.com/envoyproxy/envoy/issues/29930
 FALSE_RUNTIME_GUARD(envoy_reloadable_features_quic_defer_logging_to_ack_listener);
-// TODO(#31276): flip this to true after some test time.
-FALSE_RUNTIME_GUARD(envoy_restart_features_use_fast_protobuf_hash);
 // TODO(panting): flip this to true after some test time.
 FALSE_RUNTIME_GUARD(envoy_reloadable_features_use_config_in_happy_eyeballs);