Changing naming of counter metrics to comply with the Prometheus naming conventions

[daviddetorres]

I made a PR in Prometheus documentation repository to add this exporter to the list of exporters. In the PR the revisor (@brian-brazil) pointed out some changes that would be needed to comply with the naming conventions of Prometheus metrics.

• Add the sufix "_total" to all the counter metrics
• Change $SYS/broker/uptime and $SYS/broker/clients/maximum to type gauge

Also would be interesting to:

• Include $SYS/broker/version as a metric
• Include $SYS/broker/timestamp
• Include a metric for when the last update was received (this would be included in the PR Add metrics for monitoring the broker and connection status #22 )

The most complex here would be adding the sufix to all the counters, due to the metrics would change from the current version to this one and all the queries using this metrics would need to be updated.