Metrics cleanup

Correct typos.

Reduce construction of huge strings where possible.

Remove code that added a "metric prefix" to output as that is not
correct.

Corrected bugs that prevented cart pull metrics from working.

Now using the new BufferedLineParser to process output from cart and
app metrics.

Simplified cgroups parsing and publishing logic

Added sample metrics script to the mock cart.

Renamed MetricPlugin to MetricsPlugin