A metric is a measurement of a service captured at runtime. The moment of capturing a measurements is known as a metric event, which consists not only of the measurement itself, but also the time at which it was captured and associated metadata.
Application and request metrics are important indicators of availability and performance. Custom metrics can provide insights into how availability indicators impact user experience or the business. Collected data can be used to alert of an outage or trigger scheduling decisions to scale up a deployment automatically upon high demand.
To understand how metrics in OpenTelemetry works, let’s look at a list of components that will play a part in instrumenting our code.
A Meter Provider (sometimes called
MeterProvider) is a factory for
In most applications, a Meter Provider is initialized once and its lifecycle
matches the application’s lifecycle. Meter Provider initialization also includes
Resource and Exporter initialization. It is typically the first step in metering
with OpenTelemetry. In some language SDKs, a global Meter Provider is already
initialized for you.
A Meter creates metric instruments, capturing measurements about a service at runtime. Meters are created from Meter Providers.
Metric Exporters send metric data to a consumer. This consumer can be standard output for debugging and development-time, the OpenTelemetry Collector, or any open source or vendor backend of your choice.
In OpenTelemetry measurements are captured by metric instruments. A metric instrument is defined by:
- Unit (optional)
- Description (optional)
The name, unit, and description are chosen by the developer or defined via semantic conventions for common ones like request and process metrics.
The instrument kind is one of the following:
- Counter: A value that accumulates over time – you can think of this like an odometer on a car; it only ever goes up.
- Asynchronous Counter: Same as the Counter, but is collected once for each export. Could be used if you don’t have access to the continuous increments, but only to the aggregated value.
- UpDownCounter: A value that accumulates over time, but can also go down again. An example could be a queue length, it will increase and decrease with the number of work items in the queue.
- Asynchronous UpDownCounter: Same as the UpDownCounter, but is collected once for each export. Could be used if you don’t have access to the continuous changes, but only to the aggregated value (e.g., current queue size).
- Gauge: Measures a current value at the time it is read. An example would be the fuel gauge in a vehicle. Gauges are asynchronous.
- Histogram: A client-side aggregation of values, such as request latencies. A histogram is a good choice if you are interested in value statistics. For example: How many requests take fewer than 1s?
In addition to the metric instruments, the concept of aggregations is an important one to understand. An aggregation is a technique whereby a large number of measurements are combined into either exact or estimated statistics about metric events that took place during a time window. The OTLP protocol transports such aggregated metrics. The OpenTelemetry API provides a default aggregation for each instrument which can be overridden using the Views. The OpenTelemetry project aims to provide default aggregations that are supported by visualizers and telemetry backends.
Unlike request tracing, which is intended to capture request lifecycles and provide context to the individual pieces of a request, metrics are intended to provide statistical information in aggregate. Some examples of use cases for metrics include:
- Reporting the total number of bytes read by a service, per protocol type.
- Reporting the total number of bytes read and the bytes per request.
- Reporting the duration of a system call.
- Reporting request sizes in order to determine a trend.
- Reporting CPU or memory usage of a process.
- Reporting average balance values from an account.
- Reporting current active requests being handled.
A view provides SDK users with the flexibility to customize the metrics output by the SDK. You can customize which metric instruments are to be processed or ignored. You can also customize aggregation and what attributes you want to report on metrics.
Metrics are a stable signal in the OpenTelemetry specification. For the individual language specific implementations of the Metrics API & SDK, the status is as follows:
|Ruby||Not yet implemented|
To learn more about metrics in OpenTelemetry, see the metrics specification.