Add metrics stub

lib/kafkaex_lag_exporter/metrics.ex

@@ -0,0 +1,46 @@
+defmodule KafkaexLagExporter.Metrics do
+  @moduledoc "Metrics module is responsible for building and collecting kafka metrics"
+
+  use PromEx.Plugin
+
+  require Logger
+
+  @kafka_event :kafka
+
+  @impl true
+  def manual_metrics(_opts) do
+    clients = Application.get_env(:brod, :clients)
+    [endpoint | _] = clients[:kafka_client][:endpoints] || [{"redpanda", 29_092}]
+
+    Manual.build(
+      :application_versions_manual_metrics,
+      {__MODULE__, :kafka_metrics, [endpoint, []]},
+      [
+        last_value(
+          [@kafka_event, :consumergroup, :group, :topic, :sum, :lag],
+          event_name: [@kafka_event, :consumergroup, :group, :topic, :sum, :lag],
+          description: "Sum of group offset lag across topic partitions",
+          measurement: :lag,
+          tags: [:cluster_name, :group, :topic]
+        )
+      ]
+    )
+  end
+
+  @doc false
+  def kafka_metrics({host, _port}, consumer_lags) do
+    Enum.each(consumer_lags, fn [group_name, lag] ->
+      :telemetry.execute(
+        [@kafka_event, :consumergroup, :group, :topic, :sum, :lag],
+        %{
+          lag: lag
+        },
+        %{
+          cluster_name: host,
+          group: group_name,
+          topic: []
+        }
+      )
+    end)
+  end
+end