Add support for Datadog APM

[jyee]

Adds support for the Datadog APM.

• Adds new DD_APM_ENABLED env var and kicks off trace agent in extras/run-dogstatsd.sh
• Updates env vars for preferred DD_ styling (used by Datadog libraries and in docs), but maintains support for previous naming as well
• Updates README.md with information about tracing and available env vars.

p.s. - most of this is copied over from work done on https://github.com/Datadog/heroku-buildpack-datadog

[jyee]

Note: Don't merge yet. There's a problem with the apt-get installation which appears to be clobbering agent versions and installing the wrong one(s):

remote: -----> Fetching deb packages
remote: -----> Installing datadog-agent_1%3a5.10.0-1_amd64.deb
remote: -----> Installing datadog-agent_1%3a5.10.1-1_amd64.deb
remote: -----> Installing datadog-agent_1%3a5.11.0-1_amd64.deb
remote: -----> Installing datadog-agent_1%3a5.11.2-1_amd64.deb
remote: -----> Installing datadog-agent_1%3a5.8.5-1_amd64.deb
remote: -----> Installing datadog-agent_1%3a5.9.1-1_amd64.deb
remote: -----> Installing dd-trace-agent_0.99.1101_amd64.deb
remote: -----> Installing dd-trace-agent_0.99.156_amd64.deb
remote: -----> Installing dd-trace-agent_0.99.162_amd64.deb
remote: -----> Installing dd-trace-agent_0.99.165_amd64.deb
remote: -----> Datadog Agent package installed

This means the agent installed is listening on the old port 7777 while the new py, rb and go libraries should be sending traces on 8126

[jyee]

Ok, ready to merge again. Instead of looping through .deb packages, which was installing old versions, it now installs the most recent one. It's going based on update date rather than package name because as we can see from the listing 5.8 and 5.9 are messing with things (since they're not 5.08 and 5.09).

Side benefit, slugs are now a bit smaller without the unnecessary old packages.

[miketheman]

@jyee The original issues was reported in #4.
Does the deb package no longer pull in any other dependent packages?
The mechanism to install was looping through any deb packages that were pulled in via apt-get, and if we only select the latest one from cache, then we risk missing any other packages (hint: integrations?).