A way to add custom attributes to all metrics.

[s0]

It's quite often the case that it's desirable to add additional attributes across all metrics that are produced / exported from your services, and to include certain attributes that for example are defined in ``

See:

• How to add custom attributes to http auto instrumentation? #4088
• Instrumentation Library customization opentelemetry-specification#4311

For example, you may want to include deployment.environment.name or process.pid (which will be made available as a resource attribute automatically by @opentelemetry/sdk-node.

It's important though that all these additional attributes remain static during a process's lifetime.

An approach that we've ended up taking is to require any additional attributes we want to use be defined as resource attributes, and then allow for specifying which resource attributes you'd like to be included as attributes for all exported metrics. This approach should be relatively stable and work across all exporters:

import { NodeSDK, resources } from "@opentelemetry/sdk-node";
import { getNodeAutoInstrumentations } from "@opentelemetry/auto-instrumentations-node";
import {
  ConsoleMetricExporter,
  DataPoint,
  MetricData,
  PeriodicExportingMetricReader,
  PushMetricExporter,
  ResourceMetrics,
} from "@opentelemetry/sdk-metrics";
import { ATTR_SERVICE_NAME } from "@opentelemetry/semantic-conventions";

const extendData =
  (resource: resources.Resource, attributes: Set<string>) =>
  <T, D extends MetricData & { dataPoints: DataPoint<T>[] }>(data: D): D => ({
    ...data,
    dataPoints: data.dataPoints.map((dataPoint) => ({
      ...dataPoint,
      attributes: {
        ...dataPoint.attributes,
        ...Object.fromEntries(Object.entries(resource.attributes).filter(([key]) => attributes.has(key))),
      },
    })),
  });

const extendMetrics = (metrics: ResourceMetrics, attributes: Set<string>): ResourceMetrics => {
  const metricsMapper = extendData(metrics.resource, attributes);
  return {
    ...metrics,
    scopeMetrics: metrics.scopeMetrics.map((scopeMetric) => ({
      ...scopeMetric,
      metrics: scopeMetric.metrics.map(metricsMapper),
    })),
  };
};

const extendExporterWithResourceAttributes = (
  original: PushMetricExporter,
  attributes: Set<string>
): PushMetricExporter => {
  return new Proxy(original, {
    get(target, prop, receiver) {
      if (prop === "export") {
        const result: PushMetricExporter["export"] = (metrics, ...args) => {
          target.export(extendMetrics(metrics, attributes), ...args);
        };
        return result;
      }
      return Reflect.get(target, prop, receiver);
    },
  });
};

const openTelemetrySdk = new NodeSDK({
  resource: new resources.Resource({
    [ATTR_SERVICE_NAME]: "test-otel-app",
    "deployment.environment.name": "production",
  }),
  metricReader: new PeriodicExportingMetricReader({
    exporter: extendExporterWithResourceAttributes(
      new ConsoleMetricExporter(),
      new Set(["deployment.environment.name", "process.pid"])
    ),
  }),
  instrumentations: getNodeAutoInstrumentations(),
});

openTelemetrySdk.start();