Add instrumenter for explorer

apps/explorer/lib/explorer/prometheus/instrumenter.ex

@@ -0,0 +1,23 @@
+defmodule Explorer.Prometheus.Instrumenter do
+  @moduledoc """
+  Blocks fetch and import metrics for `Prometheus`.
+  """
+
+  use Prometheus.Metric
+
+  @histogram [
+    name: :block_import_stage_runner_duration_microseconds,
+    labels: [:stage, :runner, :step],
+    buckets: [1000, 5000, 10000, 100_000],
+    duration_unit: :microseconds,
+    help: "Block import stage, runner and step in runner processing time"
+  ]
+
+  def block_import_stage_runner(function, stage, runner, step) do
+    {time, result} = :timer.tc(function)
+
+    Histogram.observe([name: :block_import_stage_runner_duration_microseconds, labels: [stage, runner, step]], time)
+
+    result
+  end
+end