Disable shadow host suffix append (#31930)

Add runtime feature envoy.reloadable_features.enable_shadow_header_suffix to toggle appending the -shadow suffix to mirrored clusters. Default is false (appends -shadow).

Risk Level: Low
Testing: Unit
Docs Changes: Updated route mirror
Release Notes: Updated current.yaml
Platform Specific Features: n/a
Optional Runtime guard: envoy_reloadable_features_enable_shadow_header_suffix

Fixes #9094

Signed-off-by: Hochstetler, Jacob <Jacob.Hochstetler@fmr.com>
Signed-off-by: Jacob Hochstetler <jacob.hochstetler@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 35c7135d9a089d93dcd7d6e3c07d14c738223775

envoy/config/route/v3/route_components.proto

@@ -763,7 +763,8 @@ message RouteAction {
   // collected for the shadow cluster making this feature useful for testing.
   //
   // During shadowing, the host/authority header is altered such that ``-shadow`` is appended. This is
-  // useful for logging. For example, ``cluster1`` becomes ``cluster1-shadow``.
+  // useful for logging. For example, ``cluster1`` becomes ``cluster1-shadow``. This behavior can be
+  // disabled by setting ``disable_shadow_host_suffix_append`` to ``true``.
   //
   // .. note::
   //
@@ -772,7 +773,7 @@ message RouteAction {
   // .. note::
   //
   //   Shadowing doesn't support Http CONNECT and upgrades.
-  // [#next-free-field: 6]
+  // [#next-free-field: 7]
   message RequestMirrorPolicy {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.api.v2.route.RouteAction.RequestMirrorPolicy";
@@ -818,6 +819,9 @@ message RouteAction {
 
     // Determines if the trace span should be sampled. Defaults to true.
     google.protobuf.BoolValue trace_sampled = 4;
+
+    // Disables appending the ``-shadow`` suffix to the shadowed ``Host`` header. Defaults to ``false``.
+    bool disable_shadow_host_suffix_append = 6;
   }
 
   // Specifies the route's hashing policy if the upstream cluster uses a hashing :ref:`load balancer