Enable adding custom LogStasher fields from apps

I.e. allow apps to add their own custom fields in addition to those that
`GovukJsonLogging` already adds.

It looks like `LogStasher.add_custom_fields` can only be called one
time, otherwise subsequent calls overwrite previous ones.

We found this happening in the wild in the Content Store app, where
`govuk_request_id` et al were missing from the Rails framework logs but
present whenever the logger had been invoked directly from our own code.

I am curious about the possibility of being able to add custom fields to
both types of logs, together or separately, but the bug we're currently
experiencing is just a conflict between two calls to
`LogStasher.add_custom_fields`, so that's what I'm addressing.

---

I've manually tested this change locally with Content Store, both with
and without its own custom field config.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Enable adding custom LogStasher fields from apps ([#327](https://github.com/alphagov/govuk_app_config/pull/327))
+
 # 9.6.0
 
 * Allow YouTube thumbnails from https://i.ytimg.com in the global Content Security Policy ([#328](https://github.com/alphagov/govuk_app_config/pull/328))

README.md

@@ -149,6 +149,20 @@ allow JSON format logs and `Govuk-Request-Id` to be visible.
 For development logs, in order to see the production style logs, developers should
 set `GOVUK_RAILS_JSON_LOGGING`in `govuk-docker` -> `docker-compose` files.
 
+### Logger configuration
+
+To include additional custom fields in your Rails logs, you can declare them
+within a `GovukJsonLogging.configure` block in a `config/initializers/` file.
+
+Example of adding a key/value to log entries based on a request header:
+
+```ruby
+GovukJsonLogging.configure do
+  add_custom_fields do |fields|
+    fields[:govuk_custom_field] = request.headers["GOVUK-Custom-Header"]
+  end
+end
+```
 
 ## Content Security Policy generation
 

lib/govuk_app_config/govuk_json_logging.rb

@@ -3,7 +3,23 @@
 require "action_controller"
 
 module GovukJsonLogging
-  def self.configure
+  class Configuration
+    def initialize
+      @custom_fields_block = proc {}
+    end
+
+    attr_reader :custom_fields_block
+
+    def add_custom_fields(&block)
+      @custom_fields_block = block if block_given?
+    end
+  end
+
+  def self.configure(&block)
+    configuration = Configuration.new
+
+    configuration.instance_eval(&block) if block_given?
+
     # We disable buffering, so that logs aren't lost on crash or delayed
     # indefinitely while troubleshooting.
     $stdout.sync = true
@@ -31,6 +47,8 @@ def self.configure
       fields[:govuk_request_id] = request.headers["GOVUK-Request-Id"]
       fields[:varnish_id] = request.headers["X-Varnish"]
       fields[:govuk_app_config] = GovukAppConfig::VERSION
+
+      instance_exec(fields, &configuration.custom_fields_block) if block_given?
     end
 
     Rails.application.config.logstasher.enabled = true