fix(mockotlpserver): don't throw when printing a metrics summary for a histogram with no attributes

examples/some-metrics.js

@@ -0,0 +1,70 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *	http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+'use strict';
+
+// An application that creates one of each kind of custom metric for
+// demonstration purposes.
+
+const otel = require('@opentelemetry/api');
+
+const meter = otel.metrics.getMeter('some-metrics');
+
+const counter = meter.createCounter('my_counter');
+
+let n = 0;
+const asyncCounter = meter.createObservableCounter('my_async_counter');
+asyncCounter.addCallback((observableResult) => {
+    observableResult.observe(n);
+});
+
+const asyncGauge = meter.createObservableGauge('my_async_gauge');
+asyncGauge.addCallback((observableResult) => {
+    // A sine wave with a 5 minute period, to have a recognizable pattern.
+    observableResult.observe(
+        Math.sin((Date.now() / 1000 / 60 / 5) * (2 * Math.PI))
+    );
+});
+
+const upDownCounter = meter.createUpDownCounter('my_updowncounter');
+
+let c = 0;
+const asyncUpDownCounter = meter.createObservableUpDownCounter(
+    'my_async_updowncounter'
+);
+asyncUpDownCounter.addCallback((observableResult) => {
+    observableResult.observe(c);
+});
+
+const histogram = meter.createHistogram('my_histogram');
+
+setInterval(() => {
+    n++;
+    counter.add(1);
+    if (new Date().getUTCSeconds() < 30) {
+        c++;
+        upDownCounter.add(1);
+    } else {
+        c--;
+        upDownCounter.add(-1);
+    }
+    histogram.record(2);
+    histogram.record(3);
+    histogram.record(4);
+}, 200);

packages/mockotlpserver/CHANGELOG.md

@@ -1,5 +1,9 @@
 # @elastic/mockotlpserver Changelog
 
+## Unreleased
+
+- fix: Don't throw printing a metrics summary for a histogram without attributes.
+
 ## v0.5.0
 
 - fix: Add shebang line to the CLI script so `npx @elastic/mockotlpserver` works.

packages/mockotlpserver/lib/metrics-summary.js

@@ -51,12 +51,19 @@ class MetricsSummaryPrinter extends Printer {
                             rendering.push(`      ${metric.name} (histogram)`);
                         } else {
                             const dp = metric.histogram.dataPoints[0];
+                            const extras = ['histogram'];
+                            if (metric.unit) {
+                                extras.push(metric.unit);
+                            }
+                            if (dp.attributes) {
+                                extras.push(
+                                    `${Object.keys(dp.attributes).length} attrs`
+                                );
+                            }
                             rendering.push(
-                                `      ${metric.name} (histogram, ${
-                                    metric.unit
-                                }, ${
-                                    Object.keys(dp.attributes).length
-                                } attrs): min=${dp.min}, max=${dp.max}`
+                                `      ${metric.name} (${extras.join(
+                                    ', '
+                                )}): min=${dp.min}, max=${dp.max}`
                             );
                         }
                     } else {