chore: adding test-consumer documentation

src/content/docs/reference/test-consumer.md

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+---
+title: '@metrics/test-consumer'
+---
+
+This module is a memory based consumer for metrics streams to be used in tests. The purpose of the module is to make writing tests and asserting metrics easier.
+It takes a metric stream generated by @metrics/client and makes the collected metrics available as an array.
+
+⚠️ You should _never_ use this in produciton code, however it is very convenient when writing tests which produce metrics using [`@metrics/client`](https://metrics-js.github.io/reference/client/)
+
+## Examples usage
+
+Below is a sample test showing how this could be used:
+
+```js
+const test = require('tap');
+const Metrics = require('@metrics/client');
+const TestConsumer = require('@metrics/test-consumer');
+
+test('some test case', async () => {
+    const metrics = new Metrics();
+    // This sets up the metrics client to be used
+    const testHelper = new TestConsumer(metrics)
+    // .start sets up the stream
+    testHelper.start();
+
+    // Code which triggers a count metric
+
+    testHelper.stop(); // .stop ends the streams and now we can get the result.
+
+    testHelper.getResults().then(result => {
+        t.equal(result.name, "some_counter"); // Validate our metrics was collected
+        res.labels.forEach((metricLabel) => {
+            if (metricLabel.name === "value") {
+                t.equal(metricLabel.value, 2); // We expect two counts to have happened
+            }
+        });
+    });
+});
+```
+
+## API
+
+-   [constructor](#constructormetrics)
+    -   [start](#start)
+    -   [stop](#stop)
+    -  [getResults](#async-getresults)
+- [createMetrics](#createmetrics)
+
+### constructor(metrics)
+
+Takes in the [@metrics/client](https://metrics-js.github.io/reference/client/) to collect metrics from.
+
+### .start()
+
+Creates a readable stream which listens for metrics produced by the client.
+
+### .stop()
+
+Ends the stream setup for the client, returns a Promise which resolves to an array with the metrics consumed.
+
+### async .getResults()
+
+Returns a promise which resolves to an array containing collected `[@metrics/metrics](https://metrics-js.github.io/reference/metric/)` objects.
+
+### createMetrics
+
+Utility object with functions for creating mock `Metric` objects, has the functions `.timer` and `.counter`.