Avoid duplicate countitems metrics

Metrics from both workers & managers are instantiated in both kind of
processes, leading to duplicates when

Issue: S3UTILS-175

CountItems/CountManager.js

@@ -2,7 +2,7 @@ const async = require('async');
 const { once } = require('arsenal').jsutil;
 const { validStorageMetricLevels } = require('./utils/constants');
 const { consolidateDataMetrics } = require('./utils/utils');
-const monitoring = require('../utils/monitoring');
+const monitoring = require('../utils/monitoring').getManagerMetrics();
 
 class CountManager {
     constructor(params) {

CountItems/CountWorker.js

@@ -1,7 +1,7 @@
 const assert = require('assert');
 const async = require('async');
 const { BucketInfo } = require('arsenal').models;
-const monitoring = require('../utils/monitoring');
+const monitoring = require('../utils/monitoring').getWorkerMetrics();
 
 class CountWorker {
     constructor(params) {

utils/monitoring.js

@@ -5,30 +5,38 @@ const { http } = require('httpagent');
 const aggregatorRegistry = new promClient.AggregatorRegistry();
 const { collectDefaultMetrics } = promClient;
 
-// Histogram of the bucket processing duration, by the utilization service.
-const bucketProcessingDuration = new promClient.Histogram({
-    name: 's3_countitems_bucket_listing_duration_seconds',
-    help: 'Bucket processing duration',
-    buckets: [1, 10, 60, 600, 3600, 18000, 36000],
-});
+function getManagerMetrics() {
+    // Histogram of the bucket processing duration, by the utilization service.
+    const bucketProcessingDuration = new promClient.Histogram({
+        name: 's3_countitems_bucket_listing_duration_seconds',
+        help: 'Bucket processing duration',
+        buckets: [1, 10, 60, 600, 3600, 18000, 36000],
+    });
 
-const consolidationDuration = new promClient.Histogram({
-    name: 's3_countitems_bucket_merge_duration_seconds',
-    help: 'Duration of metrics consolidation in seconds',
-    buckets: [0.01, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10],
-});
+    const consolidationDuration = new promClient.Histogram({
+        name: 's3_countitems_bucket_merge_duration_seconds',
+        help: 'Duration of metrics consolidation in seconds',
+        buckets: [0.01, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10],
+    });
 
-const bucketsCount = new promClient.Counter({
-    name: 's3_countitems_total_buckets_count',
-    help: 'Total number of buckets processed',
-    labelNames: ['status'],
-});
+    return { bucketProcessingDuration, consolidationDuration };
+}
+
+function getWorkerMetrics() {
+    const bucketsCount = new promClient.Counter({
+        name: 's3_countitems_total_buckets_count',
+        help: 'Total number of buckets processed',
+        labelNames: ['status'],
+    });
 
-const objectsCount = new promClient.Counter({
-    name: 's3_countitems_total_objects_count',
-    help: 'Total number of objects processed',
-    labelNames: ['status'],
-});
+    const objectsCount = new promClient.Counter({
+        name: 's3_countitems_total_objects_count',
+        help: 'Total number of objects processed',
+        labelNames: ['status'],
+    });
+
+    return { bucketsCount, objectsCount };
+}
 
 /**
  * @param {http.ServerResponse} res - http response object
@@ -78,8 +86,6 @@ module.exports = {
     client: promClient,
     collectDefaultMetrics,
     metricsHandler,
-    bucketProcessingDuration,
-    consolidationDuration,
-    bucketsCount,
-    objectsCount,
+    getManagerMetrics,
+    getWorkerMetrics,
 };