feat: add metric service

__tests__/src/shared/metric_service.spec.js

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+import MetricService from '../../../src/shared/metric_service.js';
+
+describe('Metric Service', () => {
+  it('should throw if trying to emit an non-Metric', async () => {
+    await expect(async () => {
+      await MetricService.emit('my-made-up-metric');
+    }).rejects.toThrow(/Invalid metric/);
+    //TODO https://github.com/lfilho/ddg-test-project/issues/46
+  });
+});

src/shared/metric_service.js

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+import Metric from './model/metric.js';
+import fetch from './util/fake_fetch.js';
+
+export default class MetricService {
+  static async emit(metric) {
+    if (!(metric instanceof Metric)) {
+      throw new Error('Invalid metric.');
+    }
+
+    const metricPayload = getMetricPayload(metric);
+    const metricJson = getMetricJson(metricPayload);
+    const metricString = getMetricString(metricPayload);
+
+    // Here we would make a `fetch` request to our metric service
+    // For now, let's just log mimic an async request
+    return fetch(metricJson)
+      .then((response) => {
+        console.log(`Metric sent! Server returned status: ${response.status}`);
+        console.info(metricString);
+      })
+      .catch((error) => {
+        console.error(
+          'Error while sending metric. Did you pay your internet bill?',
+          error
+        );
+      });
+  }
+}
+
+// when are private fields and methods coming? :-(
+// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields
+
+function getMetricPayload(metric) {
+  const timestamp = new Date().toISOString();
+
+  const payload = {
+    timestamp,
+    dimension: metric.dimension,
+    value: metric.value,
+  };
+
+  return payload;
+}
+
+function getMetricJson(payload) {
+  return JSON.stringify(payload);
+}
+
+function getMetricString(payload) {
+  return `INFO  ${payload.timestamp}  ${payload.dimension}=${payload.value}`;
+}

src/shared/util/fake_fetch.js

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+// A simple `fetch` mock so we can unit test via node without extra dependencies
+
+export default async function fetch() {
+  return Promise.resolve({ status: 200 });
+}