Install the requirements
sudo apt-get install sysstat
Install the agent from Datadog "from source". You'll get a command to run on the Pi that looks like this:
DD_API_KEY=0123456789abcdef0123456789abcdef sh -c "$(curl -L https://raw.githubusercontent.com/DataDog/dd-agent/master/packaging/datadog-agent/source/setup_agent.sh)"
Run that and wait for it to install and the agent to start running.