Simple Custom Telemetry Metrics in Elixir

In a previous post I discussed how to setup your Elixir app to send metrics, logs, and traces to Grafana. In this post we will discuss adding custom metrics and traces.

Traces

Traces are very straightforward and we just do what the docs say:

require OpenTelemetry.Tracer, as: Tracer

Tracer.with_span "span-1" do
  ... do something ...
end

Metrics

Metrics are slightly more involved. First, we need to fire off :telemetry events, then we need to setup a custom PromEx plugin to report the metrics.

Let’s say I want to count the number of requests to a specific controller. I’d add this to my controller:

:telemetry.execute([:http, :request, :mycontroller], %{count: 1})

and then create a custom PromEx plugin

defmodule Observability.MycontrollerMetric do
  use PromEx.Plugin

  @impl true
  def event_metrics(opts) do
    Event.build(
      :observability_mycontroller_metrics,
      [
        Telemetry.Metrics.counter("http.request.mycontroller.count")
      ]
    )
  end
end

And finally, add it to your PromEx plugin list.

Note If you misconfigure the metric, it will not necessarily throw an error. For example, if you wrote http.request.mycontroller (missing the .count) then it will not work but it will also not crash.

That’s all!