Add support for the DD_TAGS environment variable

README.md

@@ -37,7 +37,7 @@ plugin :statsd
 
 ## Usage
 
-Ensure you have an environment variable set that points to a statsd host, then boot your puma app as usual.  Optionally you may specify a port (default is 8125). 
+Ensure you have an environment variable set that points to a statsd host, then boot your puma app as usual.  Optionally you may specify a port (default is 8125).
 
 ```
 STATSD_HOST=127.0.0.1 bundle exec puma
@@ -53,9 +53,25 @@ metric tags are a non-standard addition to the statsd protocol, supported by
 the datadog "dogstatsd" server.
 
 Should you be reporting the puma metrics to a dogstatsd server, you can set
-tags via the following two environment variables.
+tags via the following three environment variables.
 
-`MY_POD_NAME` adds a `pod_name` tag to the metrics. The `MY_POD_NAME`
+
+#### DD_TAGS
+
+`DD_TAGS`: Set this to a space-separated list of tags, using the
+[datadog agent standard format](https://docs.datadoghq.com/agent/docker/?tab=standard#global-options).
+
+For example, you could set this environment variable to set three datadog tags,
+and then you can filter by in the datadog interface:
+
+```bash
+export DD_TAGS="env:test simple-tag-0 tag-key-1:tag-value-1"
+bundle exec rails server
+```
+
+#### MY_POD_NAME
+
+`MY_POD_NAME`: Set a `pod_name` tag to the metrics. The `MY_POD_NAME`
 environment variable is recommended in the datadog kubernetes setup
 documentation, and for puma apps deployed to kubernetes it's very helpful to
 have the option to report on specific pods.
@@ -70,7 +86,9 @@ env:
         fieldPath: metadata.name
 ```
 
-`STATSD_GROUPING` adds a `grouping` tag to the metrics, with a value equal to
+#### STATSD_GROUPING
+
+`STATSD_GROUPING`: add a `grouping` tag to the metrics, with a value equal to
 the environment variable value. This is particularly helpful in a kubernetes
 deployment where each pod has a unique name but you want the option to group
 metrics across all pods in a deployment. Setting this on the pods in a
@@ -100,7 +118,7 @@ Start puma:
 Throw some traffic at it, either with curl or a tool like ab:
 
     curl http://127.0.0.1:9292/
-    ab -n 10000 -c 20 http://127.0.0.1:9292/ 
+    ab -n 10000 -c 20 http://127.0.0.1:9292/
 
 Watch the output of the UDP server process - you should see statsd data printed to stdout.
 

lib/puma/plugin/statsd.rb

@@ -20,12 +20,9 @@ def enabled?
     !!host
   end
 
-  def send(metric_name:, value:, type:, tags: {})
+  def send(metric_name:, value:, type:, tags: nil)
     data = "#{metric_name}:#{value}|#{STATSD_TYPES.fetch(type)}"
-    if tags.any?
-      tag_str = tags.map { |k,v| "#{k}:#{v}" }.join(",")
-      data = "#{data}|##{tag_str}"
-    end
+    data = "#{data}|##{tags}" unless tags.nil?
 
     socket = UDPSocket.new
     socket.send(data, 0, host, port)
@@ -123,15 +120,39 @@ def fetch_stats
     end
   end
 
-  def tags
-    tags = {}
+  def environment_variable_tags
+    # Tags are separated by spaces, and while they are normally a tag and
+    # value separated by a ':', they can also just be tagged without any
+    # associated value.
+    #
+    # Examples: simple-tag-0 tag-key-1:tag-value-1
+    #
+    tags = []
+
     if ENV.has_key?("MY_POD_NAME")
-      tags[:pod_name] = ENV.fetch("MY_POD_NAME", "no_pod")
+      tags << "pod_name:#{ENV['MY_POD_NAME']}"
     end
+
     if ENV.has_key?("STATSD_GROUPING")
-      tags[:grouping] = ENV.fetch("STATSD_GROUPING", "no-group")
+      tags << "grouping:#{ENV['STATSD_GROUPING']}"
     end
-    tags
+
+    # Standardised datadog tag attributes, so that we can share the metric
+    # tags with the application running
+    #
+    # https://docs.datadoghq.com/agent/docker/?tab=standard#global-options
+    #
+    if ENV.has_key?("DD_TAGS")
+      ENV["DD_TAGS"].split(/\s+/).each do |t|
+        tags << t
+      end
+    end
+
+    # Return nil if we have no environment variable tags. This way we don't
+    # send an unnecessary '|' on the end of each stat
+    return nil if tags.empty?
+
+    tags.join(",")
   end
 
   def prefixed_metric_name(puma_metric)
@@ -140,6 +161,8 @@ def prefixed_metric_name(puma_metric)
 
   # Send data to statsd every few seconds
   def stats_loop
+    tags = environment_variable_tags
+
     sleep 5
     loop do
       @launcher.events.debug "statsd: notify statsd"