[BUG] Peep doesn't support reporter_options.buckets by default

[ruslandoga]

👋

The way Peep storage works right now is incompatible with the rest of PromEx as Peep ignores the buckets defined for distributions (i.e. reporter_options.buckets). Maybe PromEx could include a custom Peep bucket calculator that would support reporter_options.buckets?

[ruslandoga]

Possible implementation for the custom bucket calculator :)

defmodule PromEx.PeepBuckets do
   @moduledoc """
   Adapts `Peep` for `PromEx`.
   Based on `Peep.Buckets.Custom` and `TelemetryMetricsPrometheus.Core`.
   """

   @behaviour Peep.Buckets

   @impl true
   def config(%Telemetry.Metrics.Distribution{reporter_options: reporter_options}) do
     # PromEx configures buckets with `:reporter_options`
     buckets = Keyword.fetch!(reporter_options, :buckets)

     if Enum.empty?(buckets) do
       raise ArgumentError, "expected buckets list to be non-empty, got #{inspect(buckets)}"
     end

     unless Enum.all?(buckets, &is_number/1) do
       raise ArgumentError,
             "expected buckets list to contain only numbers, got #{inspect(buckets)}"
     end

     unless buckets == Enum.uniq(Enum.sort(buckets)) do
       raise ArgumentError, "expected buckets to be ordered ascending, got #{inspect(buckets)}"
     end

     number_of_buckets = length(buckets)

     int_tree = :gb_trees.from_orddict(int_buckets(buckets, nil, 0))

     float_tree =
       Enum.map(buckets, &(&1 * 1.0))
       |> Enum.with_index()
       |> :gb_trees.from_orddict()

     upper_bound =
       buckets
       |> Enum.with_index()
       |> Map.new(fn {boundary, bucket_idx} -> {bucket_idx, to_string(boundary * 1.0)} end)

     %{
       number_of_buckets: number_of_buckets,
       int_tree: int_tree,
       float_tree: float_tree,
       upper_bound: upper_bound
     }
   end

   @impl true
   def number_of_buckets(config) do
     config.number_of_buckets
   end

   @impl true
   def bucket_for(number, config) when is_integer(number) do
     case :gb_trees.larger(number, config.int_tree) do
       {_, bucket_idx} -> bucket_idx
       :none -> config.number_of_buckets
     end
   end

   def bucket_for(number, config) when is_float(number) do
     case :gb_trees.larger(number, config.float_tree) do
       {_, bucket_idx} -> bucket_idx
       :none -> config.number_of_buckets
     end
   end

   @impl true
   def upper_bound(bucket_idx, config) do
     Map.get(config.upper_bound, bucket_idx, "+Inf")
   end

   defp int_buckets([], _prev, _counter) do
     []
   end

   defp int_buckets([curr | tail], prev, counter) do
     case ceil(curr) do
       ^prev -> int_buckets(tail, prev, counter + 1)
       curr -> [{curr, counter} | int_buckets(tail, curr, counter + 1)]
     end
   end
 end

^{Extracted from https://github.com/plausible/analytics/pull/5130}