Add environment variables to the system process metricset

[andrewkroh]

This PR adds the environment variables that were used to start the process to the data reported in the system process metricset. The data is added as a dictionary under the system.process.env key. Environment variables must be whitelisted using an array of regular expressions specified using process.env.whitelist: [] in the module config.

This feature implemented for FreeBSD, Linux, and OS X.

Sample config:

metricbeat.modules:
- module: system
  metricsets: [process]
  process.env.whitelist: ['USER', 'PATH']

[andrewkroh]

What kind of configuration options if any (like filtering, whitelist, blacklist, enabled) should we provide with this feature?

[ruflin]

I would only add process.env.enabled: true as a config option. Filtering can be done with the processors if needed. I think with the processors we should also be able to only have the variables for certain processes and remove it in all other cases etc.

I'm unsure if we should enabled it by default. On the one hand the info can be quite useful, but it can also contain quite a list of defaults which are the same for all processes.

[tsg]

@joegallo @alexbrasetvik FYI, we're going this direction for Metricbeat.

[tsg]

I'd say we should have a whitelist and the feature should be disabled by default due to privacy concerns. Secrets are often stored in env, so we have to make it hard to leak them accidentally.

[tsg]

To be clear, I mean a whitelist on the variable names, so something like process.env.variables: ["PATH"].

[andrewkroh]

I added process.env.whitelist: [] to control what environment variables are added to the system process events. It accepts a list of regular expressions. When empty or unset, no environment variables are added to the process event.

This should be updated to the the ExactMatcher from #2433 when it merges.

This PR depends on elastic/gosigar#62 and needs to be updated after that merges to put the commit hash into the glide.yaml. (This PR has been updated.)

[ruflin]

LGTM. @andrewkroh I merged elastic/gosigar#62.

[andrewkroh]

The PR is now updated with the final gosigar commit hash.