Add systemload.average to SystemPublicMetrics

Fixes gh-1865
Merge pull request #1856 from joshiste/systemLoad

* systemLoad:
  Add systemload.average to SystemPublicMetrics
Fix typo
Detect builder-style setter

Previously, a setter method that was returning the current instance was
not identified as a "setter" by the configuration processor. As a result,
builder-style APIs were not covered by the configuration metadata.

If a setter returns either void or the current class, it is now
recognized as a valid setter.

Fixes gh-1854