Optimize metadata merging and ensure no nil leakage (#7927)

Closes #7926

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: josevalim

lib/logger/lib/logger.ex

@@ -416,7 +416,7 @@ defmodule Logger do
   @type backend :: :gen_event.handler()
   @type message :: IO.chardata() | String.Chars.t()
   @type level :: :error | :info | :warn | :debug
-  @type metadata :: keyword(String.Chars.t())
+  @type metadata :: keyword()
   @levels [:error, :info, :warn, :debug]
 
   @metadata :logger_metadata
@@ -436,15 +436,23 @@ defmodule Logger do
   @spec metadata(metadata) :: :ok
   def metadata(keyword) do
     {enabled?, metadata} = __metadata__()
+    Process.put(@metadata, {enabled?, into_metadata(keyword, metadata)})
+    :ok
+  end
 
-    metadata =
-      Enum.reduce(keyword, metadata, fn
-        {key, nil}, acc -> Keyword.delete(acc, key)
-        {key, val}, acc -> Keyword.put(acc, key, val)
-      end)
+  defp into_metadata([], metadata), do: metadata
+  defp into_metadata(keyword, metadata), do: into_metadata(keyword, [], metadata)
 
-    Process.put(@metadata, {enabled?, metadata})
-    :ok
+  defp into_metadata([{key, nil} | keyword], prepend, metadata) do
+    into_metadata(keyword, prepend, :lists.keydelete(key, 1, metadata))
+  end
+
+  defp into_metadata([{key, _} = pair | keyword], prepend, metadata) do
+    into_metadata(keyword, [pair | prepend], :lists.keydelete(key, 1, metadata))
+  end
+
+  defp into_metadata([], prepend, metadata) do
+    prepend ++ metadata
   end
 
   @doc """
@@ -671,7 +679,7 @@ defmodule Logger do
   @doc false
   def __do_log__({level, config, pdict}, chardata_or_fun, metadata) when is_list(metadata) do
     %{utc_log: utc_log?, truncate: truncate, mode: mode} = config
-    metadata = [pid: self()] ++ Keyword.merge(pdict, metadata)
+    metadata = [pid: self()] ++ into_metadata(metadata, pdict)
 
     case normalize_message(chardata_or_fun, metadata) do
       {message, metadata} ->
@@ -851,7 +859,7 @@ defmodule Logger do
 
   defp normalize_message(fun, metadata) when is_function(fun, 0) do
     case fun.() do
-      {message, fun_metadata} -> {message, Keyword.merge(metadata, fun_metadata)}
+      {message, fun_metadata} -> {message, into_metadata(fun_metadata, metadata)}
       :skip -> :skip
       message -> {message, metadata}
     end

lib/logger/test/logger_test.exs

@@ -109,15 +109,15 @@ defmodule LoggerTest do
 
     assert capture_log(fn ->
              assert Logger.bare_log(:info, "ok", application: nil, module: LoggerTest) == :ok
-           end) =~ msg("application= module=LoggerTest [info]  ok")
+           end) =~ msg("module=LoggerTest [info]  ok")
   end
 
   test "metadata compile-time merge" do
     assert Logger.metadata(module: Sample) == :ok
 
     assert capture_log(fn ->
              assert Logger.log(:info, "ok", application: nil, module: CustomTest) == :ok
-           end) =~ msg("application= module=CustomTest [info]  ok")
+           end) =~ msg("module=CustomTest [info]  ok")
   end
 
   test "metadata merge when the argument function returns metadata" do
@@ -127,7 +127,7 @@ defmodule LoggerTest do
 
     assert capture_log(fn ->
              assert Logger.bare_log(:info, fun, application: nil, module: LoggerTest) == :ok
-           end) =~ msg("application= module=Function [info]  ok")
+           end) =~ msg("module=Function [info]  ok")
   end
 
   test "enable/1 and disable/1" do