refactor(cfg): add :exs resolver source; fold Otel.pick into get_cfg

Otel resolves config.exs from an explicit keyword (it runs during runtime.exs
boot, before app env exists) — the one resolution path not expressed by get_cfg.
Add an {:exs, {kw, key}} source so that case lives in the central resolver; the
bespoke 13-line pick/3 becomes a one-line delegate.

Author: markovejnovic

lib/hyper/cfg.ex

@@ -27,7 +27,8 @@ defmodule Hyper.Cfg do
   end
 
   @type source ::
-          {:runtime, atom() | {module(), atom()}}
+          {:exs, {keyword(), atom()}}
+          | {:runtime, atom() | {module(), atom()}}
           | {:toml, String.t()}
           | {:default, term()}
 
@@ -64,6 +65,10 @@ defmodule Hyper.Cfg do
   defp from({:default, value}), do: {:ok, value}
   defp from({:toml, path}), do: Hyper.Cfg.Toml.fetch(path)
 
+  # An explicit keyword list, used when config.exs is read by hand during
+  # `config/runtime.exs` boot (before it reaches app env) — see `Hyper.Cfg.Otel`.
+  defp from({:exs, {kw, key}}) when is_list(kw), do: Keyword.fetch(kw, key)
+
   defp from({:runtime, {mod, key}}) do
     case Application.get_env(:hyper, mod) do
       kw when is_list(kw) -> Keyword.fetch(kw, key)

lib/hyper/cfg/otel.ex

@@ -7,7 +7,7 @@ defmodule Hyper.Cfg.Otel do
   `config :opentelemetry_exporter`.
   """
 
-  import Hyper.Cfg, only: [fetch_cfg: 1]
+  import Hyper.Cfg, only: [get_cfg: 1]
 
   @doc "Resolve the `:opentelemetry_exporter` options, or `:none`."
   @spec exporter_options(keyword()) :: {:ok, keyword()} | :none
@@ -28,20 +28,9 @@ defmodule Hyper.Cfg.Otel do
     end
   end
 
+  # config.exs (the explicit `exs` keyword, since we run during boot) > [otel] toml.
   @spec pick(keyword(), atom(), String.t()) :: term() | nil
-  defp pick(exs, key, toml) do
-    case Keyword.fetch(exs, key) do
-      {:ok, v} ->
-        v
-
-      :error ->
-        case fetch_cfg(toml: toml),
-          do: (
-            {:ok, v} -> v
-            :error -> nil
-          )
-    end
-  end
+  defp pick(exs, key, toml), do: get_cfg(exs: {exs, key}, toml: toml, default: nil)
 
   @spec proto(term()) :: :http_protobuf | :grpc
   defp proto(p) when p in [:http_protobuf, :grpc], do: p