Moved benchmark to its own directory and added multicore testing + a README

benchmark/README.md

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+# Load testing
+
+Load testing is performed with Artillery.
+
+Install:
+
+```bash
+cd benchmark
+npm install
+```
+
+Running the tests (on one core):
+
+```bash
+cd benchmark
+npm run start
+```
+
+You can adapt the `socketio-load-test.yaml` file to increase/decrease load.
+
+Default settings are:
+
+```yaml
+  phases:
+    - duration: 20
+      arrivalRate: 2
+```
+
+which means: during 20 seconds, 2 users will be added every second (peaking at 40 simultaneous users).
+
+Important: don't go above 40 simultaneous users for Artillery, otherwise, it is Artillery that will fail to run the tests properly.
+To know, simply run "top". The "node" process for Artillery should never reach 100%.
+
+Reports are generated in `artillery_output.html`.
+
+# Multicore tests
+
+You will want to test with Artillery running on multiple cores.
+
+You can use
+
+```bash
+./artillery_multi_core.sh
+```
+
+This will trigger 4 Artillery instances in parallel.
+
+Beware, the report generated is generated for only one instance.
+
+# How to test, what to track?
+
+While testing, you can check:
+
+- CPU load of WorkAdventure API node process (it should not reach 100%)
+- Get metrics at the end of the run: `http://api.workadventure.localhost/metrics`  
+  In particular, look for:
+  ```
+  # HELP nodejs_eventloop_lag_max_seconds The maximum recorded event loop delay.
+  # TYPE nodejs_eventloop_lag_max_seconds gauge
+  nodejs_eventloop_lag_max_seconds 23.991418879
+  ```
+  This is the maximum time it took Node to process an event (you need to restart node after each test to reset this counter)
+- Generate a profiling using "node --prof" by switching the command in docker-compose.yaml:
+  ```
+      #command: yarn dev
+      command: yarn run profile
+  ```
+  Read https://nodejs.org/en/docs/guides/simple-profiling/ on how to generate a profile.
+