Metrics: Added load average to system sampling

What does this MR do?

Adds system load average to the sampling metrics

Are there points in the code the reviewer needs to double check?

Grafana import (outsite file)

Why was this MR needed?

This is useful for make correlations with current metrics and system load. We are using it for Geo to get a better picture.

What are the relevant issue numbers?

gitlab-org/gitlab-ee#560

Screenshots (if relevant)

Grafana JSON to import System Statistics Dashboard System_Statistics-1463794593151.json