Add backend and middleware for pubsub metrics

lib/api/middlewares/pubsub-metrics.js

@@ -0,0 +1,46 @@
+'use strict';
+
+const EVENT_VERSION = '1';
+
+function pubSubMetrics (pubSubMetricsBackend) {
+    if (!pubSubMetricsBackend.isEnabled()) {
+        return function pubSubMetricsDisabledMiddleware (req, res, next) { next(); };
+    }
+
+    return function pubSubMetricsMiddleware (req, res, next) {
+        const data = getEventData(req, res);
+
+        if (data.event) {
+            pubSubMetricsBackend.sendEvent(data.event, data.attributes);
+        }
+
+        return next();
+    };
+}
+
+function getEventData (req, res) {
+    const event = req.get('Carto-Event');
+    const eventSource = req.get('Carto-Event-Source');
+    const eventGroupId = req.get('Carto-Event-Group-Id');
+
+    if (!event || !eventSource) {
+        return [undefined, undefined];
+    }
+
+    const attributes = {
+        event_source: eventSource,
+        user_id: res.locals.userId,
+        response_code: res.statusCode.toString(),
+        source_domain: req.hostname,
+        event_time: new Date().toISOString(),
+        event_version: EVENT_VERSION
+    };
+
+    if (eventGroupId) {
+        attributes.event_group_id = eventGroupId;
+    }
+
+    return { event, attributes };
+}
+
+module.exports = pubSubMetrics;