Commit Message: ext_proc: allow override metadata without grpc_service

The patch introduces overrides.grpc_initial_metadata in ExtProcPerRoute.overrides 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:
          grpc_initial_metadata:
            - key: "b"
              value: "c"
            - key: "c"
              value: "c"
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: "a"
        value: "a"
      - key: "b"
        value: "b"

that allows to have:

a: a
b: c
c: c

like the following:

[2024-01-09 16:40:23.241][1967519][debug][router] [source/common/router/router.cc:731] [Tags: "ConnectionId":"0","StreamId":"5894068517272876876"] router decoding headers:
':method', 'POST'
':path', '/envoy.service.ext_proc.v3.ExternalProcessor/Process'
':authority', 'ext-proc-proxy'
':scheme', 'http'
'te', 'trailers'
'content-type', 'application/grpc'
'a', 'a'
'b', 'c'
'c', 'c'
'x-envoy-internal', 'true'
'x-forwarded-for', '172...'

Additional Description: new ext_proc per route override of inherited grpc metadata without specifying grpc_service
Risk Level: low
Testing: Unit, Integration, Config
Docs Changes: will have to update https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/ext_proc/v3/ext_proc.proto#extensions-filters-http-ext-proc-v3-extprocperroute regarding repeated config.core.v3.HeaderValue grpc_initial_metadata = 7;.

// 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.

Release Notes: see changlelogs/current.yaml
Platform Specific Features: -

Testing

$ bazel test //test/extensions/filters/http/ext_proc:config_test --config=clang --verbose_failures --test_arg="-l trace"
$ bazel test //test/extensions/filters/http/ext_proc:filter_test --config=clang --verbose_failures --test_arg="-l trace"
$ bazel test //test/extensions/filters/http/ext_proc:ext --config=clang --verbose_failures --test_arg=--gtest_filter='*/ExtProcIntegrationTest.PerRouteGrpcMetadata/*' --test_arg="-l trace"

Test infrastructure https://github.com/sitano/envoy/tree/ivan_api_502/examples/grpc-bridge:

t1$ cd ./server && go run
t2$ cd ./ext_proc && go run
t3$ envoy/bazel-bin/source/exe/envoy-static --config-path ./envoy-proxy.yaml --log-level debug --base-id 1
// t4$ envoy/bazel-bin/source/exe/envoy-static --config-path ./envoy-proxy.yaml --log-level debug --base-id 2
t5$ envoy/bazel-bin/source/exe/envoy-static --config-path ./envoy-proxy-ext_proc-2.yaml --log-level trace --base-id 3
$ CLIENT_PROXY=http://127.0.0.1:9911 ./client.py set key1 1
$ CLIENT_PROXY=http://127.0.0.1:9911 ./client.py set key2 1
$ CLIENT_PROXY=http://127.0.0.1:9911 ./client.py set key3 1
$ ss -tnp

Example

• ext_proc_metadata_per_route_example.yaml

Request variables

Key	Value
ref	cfcc37514fe247000ae91c31c00929d492031cf9
sha	6a28914
pr	31544
base-sha	f301eeb
actor	@sitano
message	ext_proc: allow override metadata without grpc_service...
started	1708166462.349855
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)