Allow non-root user to run metrics-per-process.py

[rthouvenin]

In my set-up (Ubuntu server 16.04), some of the files in /proc/<PID>/* can be read only by root. This is annoying because:

• I actually don't care about these metrics
• This prevents the whole check to run
• I don't want the sensu-client to run as root.

As a temporary solution, I deployed a copy of metrics-per-process.py where I commented out the calls that fail. Now I am wondering what would be a better solution. I am happy to contribute, but wanted to check with you what's the best approach. I can think of:

• Have a switch to turn off the metrics that are typically restricted to root. This assumes there is such a typical set, which I am not sure of at all, and also has the drawback of not solving the problem for non-typical set-ups
• Catch the error when a metric fails, and skip it instead of aborting the whole check.
• Have a regex option to filter the list of metrics to return.

The last two could be combined.

What do you think?