Group together APM service tags with subdomains

Currently we create one "service" per domain we call (see the
split_by_domain setting just above). This works fine except for cases
like Shopify or Ngrok, which use subdomains for users. In those cases,
it's more representative to group all the subdomains together under one
service.

lib/degica_datadog/tracing.rb

@@ -49,6 +49,19 @@ def init(rake_tasks: []) # rubocop:disable Metrics/AbcSize,Metrics/MethodLength
           c.tracing.instrument :httpclient, split_by_domain: true
           c.tracing.instrument :httprb, split_by_domain: true
         end
+
+        # This block is called before traces are sent to the agent, and allows
+        # us to modify or filter them.
+        Datadog::Tracing.before_flush do |trace|
+          Datadog::Tracing::Pipeline::SpanProcessor.new do |span|
+            # Group subdomains in service tags together.
+            span.service = "myshopify.com" if span.service.end_with?("myshopify.com")
+            span.service = "ngrok.io" if span.service.end_with?("ngrok.io")
+            span.service = "ngrok-free.app" if span.service.end_with?("ngrok-free.app")
+          end
+
+          trace
+        end
       end
 
       # Start a new span.