Customize the Logger metadata key name (#1267)

In certain scenarios we may want to use the Plug.RequestID multiple
times with different configuration options (maybe different headers or
different assigns).

While this is possible, the issue is that every call adds a
`:request_id` key to the Logger metadata. When using the plug muliple
times, the last call will override the metadata of the previous one.

This commit adds a new `:logger_metadata_key` option that allows users to customize
how the request ID will be added to the logger metadata. If not
provided, this option defaults to `:request_id` for backwards
compatibility.

Author: crbelaus

lib/plug/request_id.ex

@@ -47,6 +47,13 @@ defmodule Plug.RequestId do
 
           plug Plug.RequestId, assign_as: :plug_request_id
 
+    * `:logger_metadata_key` - The name of the key that will be used to store the
+      discovered or generated request id in `Logger` metadata. If not provided,
+      the request ID Logger metadata will be stored as `:request_id`. *Available
+      since v1.18.0*.
+
+          plug Plug.RequestId, logger_metadata_key: :my_request_id
+
   """
 
   require Logger
@@ -57,15 +64,16 @@ defmodule Plug.RequestId do
   def init(opts) do
     {
       Keyword.get(opts, :http_header, "x-request-id"),
-      Keyword.get(opts, :assign_as)
+      Keyword.get(opts, :assign_as),
+      Keyword.get(opts, :logger_metadata_key, :request_id)
     }
   end
 
   @impl true
-  def call(conn, {header, assign_as}) do
+  def call(conn, {header, assign_as, logger_metadata_key}) do
     request_id = get_request_id(conn, header)
 
-    Logger.metadata(request_id: request_id)
+    Logger.metadata([{logger_metadata_key, request_id}])
     conn = if assign_as, do: Conn.assign(conn, assign_as, request_id), else: conn
 
     Conn.put_resp_header(conn, header, request_id)

test/plug/request_id_test.exs

@@ -82,6 +82,20 @@ defmodule Plug.RequestIdTest do
     assert res_request_id == meta_request_id
   end
 
+  test "adds the request id to Logger metadata with the given log key" do
+    request_id = "existingidthatislongenough"
+
+    conn =
+      conn(:get, "/")
+      |> put_req_header("x-request-id", request_id)
+      |> call(logger_metadata_key: :plug_request_id)
+
+    [res_request_id] = get_resp_header(conn, "x-request-id")
+    meta_request_id = Logger.metadata()[:plug_request_id]
+    assert generated_request_id?(res_request_id)
+    assert res_request_id == meta_request_id
+  end
+
   defp generated_request_id?(request_id) do
     Regex.match?(~r/\A[A-Za-z0-9-_]+\z/, request_id)
   end