ext_proc: allow override metadata without grpc_service (#31544)

* ext_proc: allow override metadata without grpc_service

allows `overrides.metadata` in `ExtProcPerRoute` that appends and
overrides parent metadata:

      - match:
          ...
        route:
          ...
        typed_per_filter_config:
          envoy.filters.http.ext_proc:
            "@type": type.googleapis.com/envoy.extensions.filters.http.ext_proc.v3.ExtProcPerRoute
            overrides:
              metadata:
                - key: "x-ext-proc-override"
                  value: "route3-override"
                - key: "x-router3-metadata-append"
                  value: "route3-metadata-append"
    http_filters:
    - name: envoy.filters.http.ext_proc
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.http.ext_proc.v3.ExternalProcessor
      grpc_service:
        envoy_grpc:
          cluster_name: ext-proc-proxy
        timeout: 60s
        initial_metadata:
          - key: "x-ext-proc-override"
            value: "root"
          - key: "x-ext-proc-original"
            value: "root"

that allows to have:

    x-ext-proc-override: route3-override
    x-router3-metadata-append: route3-metadata-append
    x-ext-proc-original: root

---------

Signed-off-by: Ivan Prisyazhnyy <john.koepi@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 372a262894f028431cb84f3a1d361fb8fb6a2518

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

@@ -3,6 +3,7 @@ syntax = "proto3";
 package envoy.extensions.filters.http.ext_proc.v3;
 
 import "envoy/config/common/mutation_rules/v3/mutation_rules.proto";
+import "envoy/config/core/v3/base.proto";
 import "envoy/config/core/v3/grpc_service.proto";
 import "envoy/extensions/filters/http/ext_proc/v3/processing_mode.proto";
 import "envoy/type/matcher/v3/string.proto";
@@ -273,7 +274,7 @@ message ExtProcPerRoute {
 }
 
 // Overrides that may be set on a per-route basis
-// [#next-free-field: 7]
+// [#next-free-field: 8]
 message ExtProcOverrides {
   // Set a different processing mode for this route than the default.
   ProcessingMode processing_mode = 1;
@@ -301,4 +302,10 @@ message ExtProcOverrides {
   // config used. It is the prerogative of the control plane to ensure this
   // most-specific config contains the correct final overrides.
   MetadataOptions metadata_options = 6;
+
+  // Additional metadata to include into streams initiated to the ext_proc gRPC
+  // service. This can be used for scenarios in which additional ad hoc
+  // authorization headers (e.g. ``x-foo-bar: baz-key``) are to be injected or
+  // when a route needs to partially override inherited metadata.
+  repeated config.core.v3.HeaderValue grpc_initial_metadata = 7;
 }