Add --web.disable-exporter-metrics flag
Add --web.disable-exporter-metrics flag

If this flag is set, the metrics about the exporter itself (go_*,
process_*, promhttp_*) will be excluded from /metrics.

The Kingpin way of handling boolean flags makes the negative flag
wording (_dis_able) the most reasonably one.

This also refactors the flow in node_exporter.go quite a bit to avoid
mixing up the global and a local registry and to avoid re-creating a
registry even if no filtering is requested.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@soundcloud.com>

siavashs/node_exporter,derekmarcotte/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,prometheus/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter