Add flag to control system metrics

[SuperQ]

Add a boolean flag to enable system metrics. These duplicate metrics for people running the Prometheus node_exporter. But allow it to be enabled for users without node_exporter.

Signed-off-by: Ben Kochie superq@gmail.com

[shivas]

@SuperQ thanks for PR, can I suggest making default true? That way we won't break anything for people who already using those metrics?

[SuperQ]

True is the wrong value for Prometheus, as most users already have these metrics from the node_exporter.

[SuperQ]

And the metrics coming from system are nearly useless. Just exposing a bunch of load average data that isn't important.

[shivas]

@SuperQ ok, convinced :D