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 tried to add specs for this and failed. LogStasher seems to only
load these settings inside of controllers, but adding a controller to
the dummy Rails app in the `GovukJsonLogging` specs didn't magically
solve the issue.

I don't think a more full-blown Rails app would be out-of-place in this
test suite, but without more confidence that introducing one would
actually get my specs working, I'm reluctant to spend an unspecified
amount of time bashing my head against that.

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))
+
 # 9.5.0
 
 * Allow gov.uk domains to embed pages in the global Content Security Policy ([#325](https://github.com/alphagov/govuk_app_config/pull/325))

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