Added: Lograge correlation setup instructions.

docs/GettingStarted.md

@@ -1635,14 +1635,53 @@ correlation.trace_id # => 0
 correlation.span_id # => 0
 ```
 
+#### For logging in Rails applications using Lograge (recommended)
+
+After [setting up Lograge in a Rails application][https://docs.datadoghq.com/logs/log_collection/ruby/], modify the `custom_options` block in your environment configuration file (e.g. `config/environments/production.rb`) to add the trace IDs:
+
+```ruby
+config.lograge.custom_options = lambda do |event|
+  # Retrieves trace information for current thread
+  correlation = Datadog.tracer.active_correlation
+
+  {
+    # Adds IDs as tags to log output
+    :dd => {
+      :trace_id => correlation.trace_id,
+      :span_id => correlation.span_id
+    },
+    :ddsource => ["ruby"],
+    :params => event.payload[:params].reject { |k| %w(controller action).include? k }
+  }
+end
+```
+
+#### For logging in Rails applications
+
+Rails applications which are configured with a `ActiveSupport::TaggedLogging` logger can append correlation IDs as tags to log output. The default Rails logger implements this tagged logging, making it easier to add correlation tags.
+
+In your Rails environment configuration file, add the following:
+
+```ruby
+Rails.application.configure do
+  config.log_tags = [proc { Datadog.tracer.active_correlation.to_s }]
+end
+
+# Web requests will produce:
+# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Started GET "/articles" for 172.22.0.1 at 2019-01-16 18:50:57 +0000
+# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Processing by ArticlesController#index as */*
+# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206]   Article Load (0.5ms)  SELECT "articles".* FROM "articles"
+# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Completed 200 OK in 7ms (Views: 5.5ms | ActiveRecord: 0.5ms)
+```
+
 #### For logging in Ruby applications
 
 To add correlation IDs to your logger, add a log formatter which retrieves the correlation IDs with `Datadog.tracer.active_correlation`, then add them to the message.
 
-To properly correlate with Datadog logging, be sure the following is present:
+To properly correlate with Datadog logging, be sure the following is present in the log message:
 
- - `dd.trace_id=<TRACE_ID>`: Where `<TRACE_ID>` is equal to `Datadog.tracer.active_correlation.trace_id` or `0` if no trace is active.
- - `dd.span_id=<SPAN_ID>`: Where `<SPAN_ID>` is equal to `Datadog.tracer.active_correlation.span_id` or `0` if no trace is active.
+ - `dd.trace_id=<TRACE_ID>`: Where `<TRACE_ID>` is equal to `Datadog.tracer.active_correlation.trace_id` or `0` if no trace is active during logging.
+ - `dd.span_id=<SPAN_ID>`: Where `<SPAN_ID>` is equal to `Datadog.tracer.active_correlation.span_id` or `0` if no trace is active during logging.
 
 By default, `Datadog::Correlation::Identifier#to_s` will return `dd.trace_id=<TRACE_ID> dd.span_id=<SPAN_ID>`.
 
@@ -1667,24 +1706,6 @@ Datadog.tracer.trace('my.operation') { logger.warn('This is a traced operation.'
 # [2019-01-16 18:38:41 +0000][my_app][WARN][dd.trace_id=8545847825299552251 dd.span_id=3711755234730770098] This is a traced operation.
 ```
 
-#### For logging in Rails applications
-
-Rails applications which are configured with a `ActiveSupport::TaggedLogging` logger can append correlation IDs as tags to log output. The default Rails logger implements this tagged logging, making it easier to add correlation tags.
-
-In your Rails environment configuration file, add the following:
-
-```ruby
-Rails.application.configure do
-  config.log_tags = [proc { Datadog.tracer.active_correlation.to_s }]
-end
-
-# Web requests will produce:
-# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Started GET "/articles" for 172.22.0.1 at 2019-01-16 18:50:57 +0000
-# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Processing by ArticlesController#index as */*
-# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206]   Article Load (0.5ms)  SELECT "articles".* FROM "articles"
-# [dd.trace_id=7110975754844687674 dd.span_id=7518426836986654206] Completed 200 OK in 7ms (Views: 5.5ms | ActiveRecord: 0.5ms)
-```
-
 ### OpenTracing
 
 For setting up Datadog with OpenTracing, see out [Quickstart for OpenTracing](#quickstart-for-opentracing) section for details.