Add option for configuring the location of the container's host filesystem

[andrewkroh]

Commit to review: 4b48e2f (isolates these changes from those added in #2184)

This allows the system module to read metrics related to the host machine rather than the container. When using the -system.hostfs flag, directories like /proc and /sys are assumed to be mounted relative to the path given in the value. Here's an example usage.

docker run --volume=/:/hostfs:ro --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs

And here is a more restrictive example which does not mount all of the host's filesystems inside the container. system/filesystem will not be able to report metrics for the host's filesystem.

docker run --volume=/proc:/hostfs/proc:ro --volume=/sys/fs/cgroup:/hostfs/sys/fs/cgroup --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs

[ruflin]

@andrewkroh I merged #2184 so this now needs a rebase.

[ruflin]

• Is it required to have this as a flag or could we somehow enabled this also as a config option?
• In the commit message you described very nicely how it works and what the options are. This should be added to the docs.

[andrewkroh]

Rebased.

Is it required to have this as a flag or could we somehow enabled this also as a config option?

I did see your comment about this in issue #2137, but I already had it implemented it like this so I put out the PR, but I do think it could be config file option since it is so easy to set config variables on the CLI now. The one issue I ran into was figuring out how to best route the configuration value down into the module without making a global variable. Probably the best option is to write the value to ModuleConfig, but not make it configurable by the individual modules.

In the commit message you described very nicely how it works and what the options are. This should be added to the docs.

Yeah, I haven't updated any of the docs to discuss cgroups yet either. I updated the meta issue with a task for adding info about how to run Metricbeat in a container.

[ruflin]

If somehow possible, we should make all options part of the config file, especially with centralised configuration management in mind. I see the command line flags mainly for overwriting existing config options if needed. If it is a config file option, no special flag is needed as the default "overload flag options" can be used.

Alternatively metricbeat.system.hostfs could be used. I think it is fine to use the global namespace for global options?

Update: I see there is an issue with local metricsets knowning about global variables.

[ruflin]

@andrewkroh I merged this one. I would be nice if we could find a "nice" option to have it in the config file and somehow pass it to the Module.