Expose :logger's global metadata

lib/logger/lib/logger.ex

@@ -191,6 +191,10 @@ defmodule Logger do
       should be logged. Defaults to `false`. This option only has an effect
       if `:handle_otp_reports` is true.
 
+     * `:metadata` - global primary metadata to be included in all log messages.
+      Defaults to `[]`. This can be overridden at the process level with `metadata/1`
+      or each on log call as desired.
+
   For example, to configure `Logger` to redirect all Erlang messages using a
   `config/config.exs` file:
 
@@ -452,6 +456,9 @@ defmodule Logger do
 
   @doc """
   Reads the current process metadata.
+
+  This does not return the "global" logger metadata (set via the `:metadata` key in the
+  `:logger` application config), but only the process metadata.
   """
   @spec metadata() :: metadata
   def metadata() do

lib/logger/lib/logger/app.ex

@@ -34,6 +34,12 @@ defmodule Logger.App do
     primary_config = :logger.get_primary_config()
     :ok = :logger.set_primary_config(:level, default_level())
 
+    # If there is additional metadata in the :logger config, we merge it into
+    # the primary :logger metadata.
+    with [_ | _] = metadata <- Application.fetch_env!(:logger, :metadata) do
+      :ok = :logger.set_primary_config(:metadata, Enum.into(metadata, primary_config.metadata))
+    end
+
     process_level_filter = {&Logger.Utils.process_level/2, []}
     :ok = :logger.add_primary_filter(:logger_process_level, process_level_filter)
 

lib/logger/mix.exs

@@ -25,7 +25,8 @@ defmodule Logger.MixProject do
         start_options: [],
         sync_threshold: 20,
         discard_threshold: 500,
-        discard_threshold_periodic_check: 30_000
+        discard_threshold_periodic_check: 30_000,
+        metadata: []
       ]
     ]
   end

lib/logger/test/logger_test.exs

@@ -582,6 +582,17 @@ defmodule LoggerTest do
     Application.start(:logger)
   end
 
+  test "starts the application with global metadata" do
+    Application.put_env(:logger, :metadata, global_meta: :yes)
+    Logger.App.stop()
+    Application.start(:logger)
+    assert :logger.get_primary_config()[:metadata] == %{global_meta: :yes}
+  after
+    Application.put_env(:logger, :metadata, [])
+    Logger.App.stop()
+    Application.start(:logger)
+  end
+
   test "writes to stderr on bad default handler config" do
     Application.put_env(:logger, :default_handler, config: %{file: 123})
     Logger.App.stop()