initial working version, tested on ubuntu.

playbooks/roles/datadog/files/etc/yum.repo.d/datadog.repo

+[datadog]
+name = Datadog, Inc.
+baseurl = http://yum.datadoghq.com/rpm/
+enabled=1
+gpgcheck=0
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playbooks/roles/datadog/handlers/main.yml

+---
+
+- name: datadog | restart the datadog service
+  service: name=datadog-agent state=restarted
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playbooks/roles/datadog/tasks/main.yml

+---
+
+#
+# datadog
+# 
+# Overview:
+# 
+# Installs datadog 
+##
+# Dependencies:
+#
+# Example play:
+#   roles:
+#   - common
+#   - datadog
+#
+
+- name: datadog | add apt key
+  shell: apt-key adv --keyserver keyserver.ubuntu.com --recv-keys C7A7DA52
+  tags:
+    - datadog
+    - ubuntu
+  when: ansible_distribution in common_debian_variants
+
+- name: datadog | install apt repository
+  shell: echo 'deb http://apt.datadoghq.com/ unstable main' > /etc/apt/sources.list.d/datadog-source.list
+  tags:
+    - datadog
+    - ubuntu
+  when: ansible_distribution in common_debian_variants
+
+- name: datadog | add yum repo
+  copy:
+    src=etc/yum.repo.d/datdog.repo
+    dest=/etc/yum.repo.d/datdog.repo
+  tags:
+    - datadog
+    - redhat
+  when_string: ansible_distribution in common_redhat_variants
+
+- name: datadog | install datadog agent
+  apt: pkg="datadog-agent" update_cache=yes
+  tags:
+    - datadog
+    - ubuntu
+  when: ansible_distribution in common_debian_variants
+
+- name: datadog | bootstrap config
+  shell: cp /etc/dd-agent/datadog.conf.example /etc/dd-agent/datadog.conf creates=/etc/dd-agent/datadog.conf
+  tags:
+    - datadog
+
+# quoting intentional, missing space after line=api_key: also
+# ansible wasn't handling the double quoted yaml properly 
+# otherwise.
+- name: datadog | update api-key
+  lineinfile: 
+    dest="/etc/dd-agent/datadog.conf" 
+    "regexp=^api_key:$" 
+    "line=api_key:{{ common_datadog_api_key }}"
+  notify:
+    - datadog | restart the datadog service
+  tags:
+    - datadog
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playbooks/roles/datadog/vars/main.yml

+---
+
+datadog_api_key: "PUT_YOUR_API_KEY_HERE"
+
+datadog_debian_pkgs:
+  - apparmor-utils
+  - build-essential
+  - curl
+  - g++
+  - gcc
+  - ipython
+  - pkg-config
+  - rsyslog
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