feat: add suport for registry types and exemplars

- Added support for selecting the registry type
- Added support for OpenMetrics format
- Added support for out of the box integration with OpenTelemetry so
  exemplars of the default metrics are populated with trace data

Author: Andrei Dobre

package.json

@@ -34,7 +34,8 @@
   },
   "dependencies": {
     "response-time": "^2.3.2",
-    "url-value-parser": "^2.0.0"
+    "url-value-parser": "^2.0.0",
+    "@opentelemetry/api": "^1.0.2"
   },
   "peerDependencies": {
     "express": "4.x",

src/index.js

@@ -1,5 +1,6 @@
 const express = require('express');
-const Prometheus = require('prom-client');
+const OtelApi = require('@opentelemetry/api');
+const Prometheus = require('/home/andrei/dev/prom-client');
 const ResponseTime = require('response-time');
 
 const {
@@ -20,6 +21,8 @@ const defaultOptions = {
   authenticate: null,
   collectDefaultMetrics: true,
   collectGCMetrics: false,
+  enableExemplars: false,
+  contentType: Prometheus.Registry.contentType,
   // buckets for response time from 0.05s to 2.5s
   // these are arbitrary values since i dont know any better ¯\_(ツ)_/¯
   requestDurationBuckets: Prometheus.exponentialBuckets(0.05, 1.75, 8),
@@ -41,24 +44,30 @@ module.exports = (userOptions = {}) => {
   const app = express();
   app.disable('x-powered-by');
 
+  Prometheus.register.setContentType(options.contentType);
+
   const requestDuration = requestDurationGenerator(
     options.customLabels,
     options.requestDurationBuckets,
     options.prefix,
+    options.enableExemplars,
   );
   const requestCount = requestCountGenerator(
     options.customLabels,
     options.prefix,
+    options.enableExemplars,
   );
   const requestLength = requestLengthGenerator(
     options.customLabels,
     options.requestLengthBuckets,
     options.prefix,
+    options.enableExemplars,
   );
   const responseLength = responseLengthGenerator(
     options.customLabels,
     options.responseLengthBuckets,
     options.prefix,
+    options.enableExemplars,
   );
 
   /**
@@ -73,6 +82,18 @@ module.exports = (userOptions = {}) => {
     const route = normalizePath(originalUrl, options.extraMasks);
 
     if (route !== metricsPath) {
+      let exemplarLabels = null;
+      if(options.enableExemplars) {
+        exemplarLabels = {};
+        let current_span = OtelApi.trace.getSpan(OtelApi.context.active());
+        if(current_span) {
+          exemplarLabels = {
+            'traceId': current_span.spanContext().traceId,
+            'spanId': current_span.spanContext().spanId
+          };
+        }
+      }
+
       const status = normalizeStatus
         ? normalizeStatusCode(res.statusCode) : res.statusCode.toString();
 
@@ -81,24 +102,40 @@ module.exports = (userOptions = {}) => {
       if (typeof options.transformLabels === 'function') {
         options.transformLabels(labels, req, res);
       }
-      requestCount.inc(labels);
+      if (exemplarLabels != null) {
+        requestCount.inc({ labels: labels, exemplarLabels: exemplarLabels });
+      } else {
+        requestCount.inc(labels, exemplarLabels);
+      }
 
       // observe normalizing to seconds
-      requestDuration.observe(labels, time / 1000);
+      if (exemplarLabels != null) {
+        requestDuration.observe({ labels: labels, value: (time/1000), exemplarLabels: exemplarLabels });
+      } else {
+        requestDuration.observe(labels, time / 1000);
+      }
 
       // observe request length
       if (options.requestLengthBuckets.length) {
         const reqLength = req.get('Content-Length');
         if (reqLength) {
-          requestLength.observe(labels, Number(reqLength));
+          if (exemplarLabels != null) {
+            requestLength.observe({ labels: labels, value: Number(reqLength), exemplarLabels: exemplarLabels });
+          } else {
+            requestLength.observe(labels, Number(reqLength));
+          }
         }
       }
 
       // observe response length
       if (options.responseLengthBuckets.length) {
         const resLength = res.get('Content-Length');
         if (resLength) {
-          responseLength.observe(labels, Number(resLength));
+          if (exemplarLabels != null) {
+            responseLength.observe({ labels: labels, value: Number(resLength), exemplarLabels: exemplarLabels });
+          } else {
+            responseLength.observe(labels, Number(resLength));
+          }
         }
       }
     }
@@ -110,6 +147,7 @@ module.exports = (userOptions = {}) => {
     // used to calculate saturation of the service
     Prometheus.collectDefaultMetrics({
       prefix: options.prefix,
+      enableExemplars: options.enableExemplars,
     });
   }
 
@@ -125,6 +163,7 @@ module.exports = (userOptions = {}) => {
       /* eslint-enable global-require */
       const startGcStats = gcStats(Prometheus.register, {
         prefix: options.prefix,
+        enableExemplars: options.enableExemplars
       });
       startGcStats();
     } catch (err) {
@@ -154,7 +193,7 @@ module.exports = (userOptions = {}) => {
       }
     }
 
-    res.set('Content-Type', Prometheus.register.contentType);
+    res.set('Content-Type', options.contentType);
     return res.end(await Prometheus.register.metrics());
   });
 

src/metrics.js

@@ -1,13 +1,15 @@
-const Prometheus = require('prom-client');
+// const Prometheus = require('prom-client');
+const Prometheus = require('/home/andrei/dev/prom-client');
 
 /**
  * @param prefix - metrics name prefix
  * request counter
  */
-function requestCountGenerator(labelNames, prefix = '') {
+function requestCountGenerator(labelNames, prefix = '', enableExemplars = false) {
   return new Prometheus.Counter({
     name: `${prefix}http_requests_total`,
     help: 'Counter for total requests received',
+    enableExemplars: enableExemplars,
     labelNames,
   });
 }
@@ -17,10 +19,11 @@ function requestCountGenerator(labelNames, prefix = '') {
  * @param prefix - metrics name prefix
  * request duration
  */
-function requestDurationGenerator(labelNames, buckets, prefix = '') {
+function requestDurationGenerator(labelNames, buckets, prefix = '', enableExemplars = false) {
   return new Prometheus.Histogram({
     name: `${prefix}http_request_duration_seconds`,
     help: 'Duration of HTTP requests in seconds',
+    enableExemplars: enableExemplars,
     labelNames,
     buckets,
   });
@@ -31,10 +34,11 @@ function requestDurationGenerator(labelNames, buckets, prefix = '') {
  * @param prefix - metrics name prefix
  * request length
  */
-function requestLengthGenerator(labelNames, buckets, prefix = '') {
+function requestLengthGenerator(labelNames, buckets, prefix = '', enableExemplars = false) {
   return new Prometheus.Histogram({
     name: `${prefix}http_request_length_bytes`,
     help: 'Content-Length of HTTP request',
+    enableExemplars: enableExemplars,
     labelNames,
     buckets,
   });
@@ -45,10 +49,11 @@ function requestLengthGenerator(labelNames, buckets, prefix = '') {
  * @param prefix - metrics name prefix
  * response length
  */
-function responseLengthGenerator(labelNames, buckets, prefix = '') {
+function responseLengthGenerator(labelNames, buckets, prefix = '', enableExemplars = false) {
   return new Prometheus.Histogram({
     name: `${prefix}http_response_length_bytes`,
     help: 'Content-Length of HTTP response',
+    enableExemplars: enableExemplars,
     labelNames,
     buckets,
   });