Add k6 README

Author: arelra

dotcom-rendering/scripts/perf/k6/README.md

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+# Load Testing with k6
+
+[Grafana k6](https://k6.io/) is a tool that will run a load test scenario against an endpoint and once complete provide detailed stats on latency and throughput metrics.
+
+## Installing
+
+From the k6 directory run
+
+```
+./k6-install.sh
+```
+
+Once finished you will see an example of how to run the script.
+
+## Script
+
+From the k6 directory run (for example with v0.58.0)
+
+```
+./k6-v0.58.0-macos-arm64/k6 run k6.mjs
+```
+
+The current script `k6.mjs` loads a locally saved JSON payload which will then POST to the DCR `/Article` endpoint. The local payload file removes the fetching of the JSON payload from the test scenario.
+
+You can of course configure this to be any payload and endpoint you wish.
+
+## Testing Advice
+
+Run the script 2-3 times to warm the app and instance if working from a cold start.
+
+k6 can consume lots of memory, so ideally you shouldn't run it on the same compute as the app you are load testing.
+
+k6 has a large API so its worth reading the docs and in particular the [overview](https://grafana.com/docs/k6/latest/).
+
+## Output
+
+Once complete k6 will output statistics like the following.
+
+`http_req_duration` are the main stats to consider.
+
+`iteration_duration` includes the time for k6 to run the script.
+
+```
+         /\      Grafana   /‾‾/
+    /\  /  \     |\  __   /  /
+   /  \/    \    | |/ /  /   ‾‾\
+  /          \   |   (  |  (‾)  |
+ / __________ \  |_|\_\  \_____/
+
+     execution: local
+        script: k6.mjs
+        output: -
+
+     scenarios: (100.00%) 1 scenario, 10 max VUs, 1m30s max duration (incl. graceful stop):
+              * default: 10 looping VUs for 1m0s (gracefulStop: 30s)
+
+
+
+  █ TOTAL RESULTS
+
+    checks_total.......................: 3786    62.955288/s
+    checks_succeeded...................: 100.00% 3786 out of 3786
+    checks_failed......................: 0.00%   0 out of 3786
+
+    ✓ is status 200
+
+    HTTP
+    http_req_duration.......................................................: avg=155.45ms min=54.91ms med=128.6ms  max=2.23s p(90)=250.46ms p(95)=262.3ms
+      { expected_response:true }............................................: avg=155.45ms min=54.91ms med=128.6ms  max=2.23s p(90)=250.46ms p(95)=262.3ms
+    http_req_failed.........................................................: 0.00%  0 out of 3786
+    http_reqs...............................................................: 3786   62.955288/s
+
+    EXECUTION
+    iteration_duration......................................................: avg=158.65ms min=57.77ms med=131.66ms max=2.24s p(90)=253.37ms p(95)=265.44ms
+    iterations..............................................................: 3786   62.955288/s
+    vus.....................................................................: 10     min=10        max=10
+    vus_max.................................................................: 10     min=10        max=10
+
+    NETWORK
+    data_received...........................................................: 1.1 GB 19 MB/s
+    data_sent...............................................................: 251 MB 4.2 MB/s
+
+
+
+
+running (1m00.1s), 00/10 VUs, 3786 complete and 0 interrupted iterations
+default ✓ [======================================] 10 VUs  1m0s
+```
+
+## Testing in CODE
+
+If testing in CODE:
+
+-   run the script from a detached instance
+-   k6 may get killed by the OOM killer if an instance runs out of memory. First stop the DCR process using `systemctl stop {app-name}` where app-name is `article-rendering`, `facia-rendering` etc to free up memory. If the process is repeatedly killed, run from a larger instance type with more memory or run with a smaller number of VUs.
+-   change the script endpoint to the loadbalancer of the stack you are interested in