Metrics

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.

Use BufferedLineParser to process output from cart and app metrics.

Simplify cgroups parsing and publishing logic.

Add sample metrics script to the mock and mock-plugin carts.

Rename MetricPlugin to MetricsPlugin.

Add pull metrics timeouts.

Batch cgroups data output.

Make metric metadata customizable.

Require explicit enabling of metrics plugin for it to run.

Allow user to specify cgroups metrics.

Add quota metrics.

Add unit and functional tests.