feat(examples): add rpc example with twirp

examples/rpc-twirp-with-custom-function/README.md

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+# Load testing an RPC service created with Twirp
+
+[Twirp](https://github.com/twitchtv/twirp) is a simple RPC framework for service-to-service communication. In the following example, we will use the clients auto-generated from [Twirpscript](https://github.com/tatethurston/TwirpScript), a NodeJS implementation of Twirp, to load test a server using Twirp.
+
+While we could build [a dedicated engine](https://www.artillery.io/blog/extend-artillery-by-creating-your-own-engines), you can also use custom functions and leverage the existing default engine. This example shows you how to do that.
+
+## Pre-requisites
+
+- Protobuf [installed](https://github.com/tatethurston/TwirpScript?tab=readme-ov-file#installation-)
+
+## How the example works
+
+This example imports the auto-generated `.pb.js` file into a processor, and calls the `MakeHat` client method, which will call the server. We also emit [custom metrics](https://www.artillery.io/docs/reference/extension-apis#custom-metrics-api) from the function to track the number of requests and responses, as well as the time taken in the RPC call.
+
+```
+twirp.requests: ................................................................ 300
+twirp.response_time:
+  min: ......................................................................... 1.1
+  max: ......................................................................... 60.7
+  mean: ........................................................................ 5.4
+  median: ...................................................................... 2.2
+  p95: ......................................................................... 22.4
+  p99: ......................................................................... 55.2
+twirp.responses: ............................................................... 300
+twirp.responses.success: ....................................................... 300
+```
+
+## Running the Twirp server
+
+First, install the dependencies:
+```
+cd twirp && npm install
+```
+
+Then, start the server with `npm start`
+
+## Running Artillery test
+
+Once the server is up and running, execute the test script:
+
+```
+npx artillery run ./test/scenario.yml
+```

examples/rpc-twirp-with-custom-function/test/processor.mjs

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+import { MakeHat } from '../twirp/protos/haberdasher.pb.js';
+import { client } from 'twirpscript';
+
+client.baseURL = 'http://localhost:8080';
+
+function recordMetrics(startedAt, ee, error) {
+  //you can add more domain specific metrics here dependant on the response
+  ee.emit('counter', 'twirp.requests', 1);
+  ee.emit('counter', 'twirp.responses', 1);
+  if (error) {
+    ee.emit('counter', 'twirp.responses.error', 1);
+    ee.emit('counter', 'twirp.codes.' + error.code, 1);
+  } else {
+    ee.emit('counter', 'twirp.responses.success', 1);
+  }
+
+  const took = Number(process.hrtime.bigint() - startedAt) / 1e6;
+  ee.emit('histogram', 'twirp.response_time', took);
+}
+
+export async function callRpcServer(context, ee, next) {
+  const startedAt = process.hrtime.bigint();
+  try {
+    const res = await MakeHat({ inches: 15, potato: true });
+    console.log(res);
+    recordMetrics(startedAt, ee);
+  } catch (error) {
+    recordMetrics(startedAt, ee, error);
+  }
+}

examples/rpc-twirp-with-custom-function/test/scenario.yml

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+config:
+  target: "http://localhost:8080"
+  phases:
+    - duration: 10
+      arrivalRate: 30
+      name: "Phase 1"
+  processor: "./processor.mjs"
+
+scenarios:
+  - flow:
+      - function: "callRpcServer"

examples/rpc-twirp-with-custom-function/twirp/package.json

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+{
+  "name": "twirp",
+  "version": "1.0.0",
+  "description": "",
+  "type": "module",
+  "scripts": {
+    "start": "node ./server/index.js"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC"
+}