test: add fake gnmi server to allow for local emulation

Author: felix-kaestner

test/gnmi/.gitignore

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+# SPDX-FileCopyrightText: 2025 SAP SE or an SAP affiliate company and IronCore contributors
+# SPDX-License-Identifier: Apache-2.0
+
+*
+!.gitignore
+!Dockerfile
+!README.md
+!config.pb.txt

test/gnmi/Dockerfile

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+# syntax=docker/dockerfile:1
+# SPDX-FileCopyrightText: 2025 SAP SE or an SAP affiliate company and IronCore contributors
+# SPDX-License-Identifier: Apache-2.0
+
+ARG GO_VERSION=1.24
+ARG ALPINE_VERSION=3.22
+
+FROM --platform=${BUILDPLATFORM:-linux/amd64} golang:${GO_VERSION}-alpine${ALPINE_VERSION} AS build
+
+ARG TARGETOS
+ARG TARGETARCH
+
+# Clone the openconfig/gnmi repository containing the fake server
+RUN apk add --no-cache --no-progress git && \
+    git clone --depth=1 --filter=blob:none https://github.com/openconfig/gnmi.git /go/src/github.com/openconfig/gnmi && \
+    apk del --no-cache --no-progress apk-tools alpine-keys alpine-release libc-utils
+
+# Switch into workspace
+WORKDIR /go/src/github.com/openconfig/gnmi
+
+# Install dependencies
+RUN --mount=type=cache,target=/go/pkg/mod \
+    go mod download -x && \
+    go mod tidy
+
+# Build the application into a static executable while removing the symbol table and debugging information
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags="-s -w" -o ./build/server ./testing/fake/gnmi/cmd/fake_server
+
+FROM alpine:${ALPINE_VERSION}
+
+# Switch into workspace
+WORKDIR /app
+
+# Copy executable from build
+COPY --from=build /go/src/github.com/openconfig/gnmi/build/server  /app/
+
+# Start the server application
+CMD ["/app/server", "--config=/config.pb.txt", "--text", "--port=9339", "--server_crt=/cert.pem", "--server_key=/key.pem", "--allow_no_client_auth", "--logtostderr"]

test/gnmi/README.md

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+# Fake GNMI Test Server
+
+This is a fake GNMI server that can be used to test GNMI clients.
+
+Server Configuration can be found [`config.pb.txt`](./config.pb.txt), bootstrapped from [gen_fake_config](https://github.com/openconfig/gnmi/tree/master/testing/fake/gnmi/cmd/gen_fake_config).
+
+## Prerequisites
+
+- [Docker](https://docs.docker.com/get-docker/)
+- [OpenSSL](https://www.openssl.org/)
+
+## Build
+
+All the commands below should be executed in the directory containing this `README.md` file.
+
+Generate a x509 certificate and private key for the server:
+
+```sh
+openssl req -x509 -sha256 -newkey rsa:2048 -keyout key.pem -out cert.pem -days 365 -nodes -subj "/O=SAP SE/OU=GCID PlusOne/CN=localhost" -addext "subjectAltName = DNS:localhost"
+```
+
+Build the fake GNMI server:
+
+```sh
+docker build -t ghcr.io/ironcore-dev/gnmi-test-server .
+```
+
+## Run
+
+Run the fake GNMI server:
+
+```sh
+docker run -d -p 9339:9339 -v $(pwd)/config.pb.txt:/config.pb.txt -v $(pwd)/key.pem:/key.pem -v $(pwd)/cert.pem:/cert.pem ghcr.io/ironcore-dev/gnmi-test-server
+```

test/gnmi/config.pb.txt

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+target: "targetclient"
+seed: 12345
+values: {
+  path: "openconfig"
+  path: "parent"
+  path: "child"
+  path: "state"
+  repeat: 3
+  string_value: {
+    value: "value1"
+  }
+}
+client_type: GRPC_GNMI