Revamp Docker check, more metrics, less bugs, support of CoreOS

[LotharSee]

Revamp the Docker check and its configuration.

First, it fixes:

• URLError: <urlopen error timed out> #1031 : catch the exception
• More Docker metrics about containers and images #1027: collect 3 new metrics: containers.stopped, images.available and images.intermediate
• Docker integration - AttributeError: 'NoneType' object has no attribute 'endswith' #1017 : is handled by the new logic
• Docker metrics when host is CoreOS #1001 : allow to make Docker check work inside a container. The documentation of https://github.com/DataDog/docker-dd-agent/ will need an update. For that, we will have to mount /proc/mounts and /sys/fs/cgoup inside a directory configuration as docker_root.

But also:

• Now we can add optional tags in the configuration
• Clarify configuration example
• Support CoreOS

It is also a bit more flexible (if we want to support a new cgroup hierarchy) with better logging/warning.

[remh]

Will the default behavior work with existing users that don't have this new parameter set?

[LotharSee]

Yeah, this parameter is useful when the Agent run inside a container. When we do that, we can mount some part of the host filesystem to a directory, which should be this docker_root. Then we use it as the root for our Docker daemon. By default, it is /, which means that we keep the normal behavior.

[remh]

Thanks it looks great!

What's the behavior on Docker 0.9 now ?

[LotharSee]

The error got replaced by a more generic one:
"Cannot find Docker cgroup directory. Be sure your system is supported."

So that we will be able to detail it in our documentation.

[remh]

👍

[andyshinn]

Very cool! Excited to try out the CoreOS support when 5.0 drops. Thank you!