Add source to email subject line,

services/mail.rb

@@ -25,7 +25,7 @@ def mail_message
       mail = ::Mail.new
       mail.from    'Librato Metrics <[email protected]>'
       mail.to      settings[:addresses].to_s.split(/,/).map { |a| a.strip }
-      mail.subject %{[Librato Metrics] Metric #{payload[:metric][:name]} has triggered an alert!}
+      mail.subject %{[Librato Metrics] Metric #{payload[:metric][:name]} source #{payload[:measurement][:source]} has triggered an alert!}
 
       text = text_email
       html = html_email