feat(examples): add gRPC interop example

Add examples/grpc-interop showing that any standard gRPC client can
call a go-micro service — no go-micro SDK required on the client
side. Includes:

- proto/greeter.proto with generated Go, gRPC, and micro stubs
- server/ using go-micro gRPC transport
- client/ using stock google.golang.org/grpc (no go-micro imports)
- README with Python example and explanation of how routing works

Addresses the confusion from issue #2818 where users didn't know
that go-micro gRPC services are callable by any gRPC client.

Author: claude

README.md

@@ -275,6 +275,7 @@ Every service gets `Client()`, `Server()`, and `Model()` — call services, hand
 
 Check out [/examples](examples/) for runnable code:
 - [hello-world](examples/hello-world/) - Basic RPC service
+- [grpc-interop](examples/grpc-interop/) - Call go-micro from any gRPC client
 - [web-service](examples/web-service/) - HTTP REST API
 - [multi-service](examples/multi-service/) - Multiple services in one binary
 - [mcp](examples/mcp/) - MCP integration with AI agents

examples/grpc-interop/README.md

@@ -0,0 +1,66 @@
+# gRPC Interop Example
+
+This example shows that **any standard gRPC client** can call a go-micro service — no go-micro SDK required on the client side.
+
+The server is a normal go-micro service using the gRPC transport. The client is a plain `google.golang.org/grpc` client with no go-micro imports.
+
+## How it works
+
+go-micro's gRPC server uses a [`grpc.UnknownServiceHandler`](https://pkg.go.dev/google.golang.org/grpc#UnknownServiceHandler) that catches all incoming requests and routes them by parsing the standard gRPC method path (`/package.Service/Method`). This means any language with gRPC support (Python, Java, Rust, etc.) can call go-micro services using generated protobuf stubs.
+
+## Running
+
+Start the server:
+
+```bash
+cd examples/grpc-interop
+go run ./server/
+```
+
+In another terminal, call it with the standard gRPC client:
+
+```bash
+cd examples/grpc-interop
+go run ./client/ --name Alice
+# Response: Hello Alice
+```
+
+## Calling from other languages
+
+Generate stubs from `proto/greeter.proto` in your language of choice and point them at `localhost:50051`. For example, with Python:
+
+```bash
+pip install grpcio-tools
+python -m grpc_tools.protoc -Iproto --python_out=. --grpc_python_out=. proto/greeter.proto
+```
+
+```python
+import grpc
+import greeter_pb2
+import greeter_pb2_grpc
+
+channel = grpc.insecure_channel("localhost:50051")
+stub = greeter_pb2_grpc.GreeterStub(channel)
+
+response = stub.Hello(greeter_pb2.HelloRequest(name="Alice"))
+print(response.message)  # Hello Alice
+```
+
+## Key points
+
+- The go-micro server registers handlers via `pb.RegisterGreeterHandler()`
+- The standard gRPC client uses stubs generated by `protoc-gen-go-grpc`
+- Both share the same `.proto` file — that's the contract
+- The server uses protobuf encoding on the wire, same as any gRPC service
+- Service discovery (mDNS, consul, etc.) is only needed for go-micro-to-go-micro calls; direct gRPC clients connect by address
+
+## Regenerating proto stubs
+
+```bash
+protoc --go_out=. --go_opt=paths=source_relative \
+       --go-grpc_out=. --go-grpc_opt=paths=source_relative \
+       --micro_out=. --micro_opt=paths=source_relative \
+       proto/greeter.proto
+```
+
+Requires `protoc-gen-go`, `protoc-gen-go-grpc`, and `protoc-gen-micro`.

examples/grpc-interop/client/main.go

@@ -0,0 +1,43 @@
+// client calls the go-micro Greeter service using a standard gRPC
+// client — no go-micro SDK. This proves any language with gRPC support
+// can call go-micro services.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"time"
+
+	pb "example/proto"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+)
+
+func main() {
+	addr := flag.String("addr", "localhost:50051", "server address")
+	name := flag.String("name", "World", "name to greet")
+	flag.Parse()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	conn, err := grpc.NewClient(*addr,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+	)
+	if err != nil {
+		log.Fatalf("connect: %v", err)
+	}
+	defer conn.Close()
+
+	client := pb.NewGreeterClient(conn)
+
+	resp, err := client.Hello(ctx, &pb.HelloRequest{Name: *name})
+	if err != nil {
+		log.Fatalf("Hello: %v", err)
+	}
+
+	fmt.Printf("Response: %s\n", resp.Message)
+}

examples/grpc-interop/go.mod

@@ -0,0 +1,72 @@
+module example
+
+go 1.24
+
+require (
+	go-micro.dev/v5 v5.16.0
+	google.golang.org/grpc v1.71.1
+	google.golang.org/protobuf v1.36.6
+)
+
+require (
+	dario.cat/mergo v1.0.2 // indirect
+	github.com/armon/go-metrics v0.4.1 // indirect
+	github.com/bitly/go-simplejson v0.5.0 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/coreos/go-semver v0.3.0 // indirect
+	github.com/coreos/go-systemd/v22 v22.3.2 // indirect
+	github.com/cornelk/hashmap v1.0.8 // indirect
+	github.com/cpuguy83/go-md2man/v2 v2.0.5 // indirect
+	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
+	github.com/fatih/color v1.16.0 // indirect
+	github.com/fsnotify/fsnotify v1.6.0 // indirect
+	github.com/go-redis/redis/v8 v8.11.5 // indirect
+	github.com/gogo/protobuf v1.3.2 // indirect
+	github.com/golang/protobuf v1.5.4 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/hashicorp/consul/api v1.32.1 // indirect
+	github.com/hashicorp/errwrap v1.1.0 // indirect
+	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
+	github.com/hashicorp/go-hclog v1.5.0 // indirect
+	github.com/hashicorp/go-immutable-radix v1.3.1 // indirect
+	github.com/hashicorp/go-multierror v1.1.1 // indirect
+	github.com/hashicorp/go-rootcerts v1.0.2 // indirect
+	github.com/hashicorp/golang-lru v1.0.2 // indirect
+	github.com/hashicorp/serf v0.10.1 // indirect
+	github.com/klauspost/compress v1.18.0 // indirect
+	github.com/lib/pq v1.10.9 // indirect
+	github.com/mattn/go-colorable v0.1.13 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/miekg/dns v1.1.50 // indirect
+	github.com/mitchellh/go-homedir v1.1.0 // indirect
+	github.com/mitchellh/hashstructure v1.1.0 // indirect
+	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/nats-io/nats.go v1.42.0 // indirect
+	github.com/nats-io/nkeys v0.4.11 // indirect
+	github.com/nats-io/nuid v1.0.1 // indirect
+	github.com/oxtoacart/bpool v0.0.0-20190530202638-03653db5a59c // indirect
+	github.com/patrickmn/go-cache v2.1.0+incompatible // indirect
+	github.com/pkg/errors v0.9.1 // indirect
+	github.com/rabbitmq/amqp091-go v1.10.0 // indirect
+	github.com/russross/blackfriday/v2 v2.1.0 // indirect
+	github.com/urfave/cli/v2 v2.27.6 // indirect
+	github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1 // indirect
+	go.etcd.io/bbolt v1.4.0 // indirect
+	go.etcd.io/etcd/api/v3 v3.5.21 // indirect
+	go.etcd.io/etcd/client/pkg/v3 v3.5.21 // indirect
+	go.etcd.io/etcd/client/v3 v3.5.21 // indirect
+	go.uber.org/multierr v1.10.0 // indirect
+	go.uber.org/zap v1.27.0 // indirect
+	golang.org/x/crypto v0.37.0 // indirect
+	golang.org/x/exp v0.0.0-20250305212735-054e65f0b394 // indirect
+	golang.org/x/mod v0.24.0 // indirect
+	golang.org/x/net v0.38.0 // indirect
+	golang.org/x/sync v0.13.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+	golang.org/x/text v0.24.0 // indirect
+	golang.org/x/tools v0.31.0 // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20250324211829-b45e905df463 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20250324211829-b45e905df463 // indirect
+)
+
+replace go-micro.dev/v5 => ../..