Current issues and table size metrics

[pauljwil]

I'm working on an exporter that incorporates the metrics in this project, and I extended the RethinkDB metrics to include two new types of metrics:

• the number of each of the current issue types in the current issues table
• the size (in MB) of each table.

I've already added these metrics to a fork of this repo, so if anyone here is interested, I could open a PR with these changes.

Here is a little more detail on the metrics that I'm proposing:

	// current_issues table metrics
	e.metrics.logWriteIssues = prometheus.NewDesc(
		"log_write_issues",
		"Number of log write issues",
		nil, nil,
	)
	e.metrics.nameCollisionIssues = prometheus.NewDesc(
		"name_collision_issues",
		"Number of name collision issues",
		nil, nil,
	)
	e.metrics.outdatedIndexIssues = prometheus.NewDesc(
		"outdated_index_issues",
		"Number of outdated index issues",
		nil, nil,
	)
	e.metrics.totalAvailabilityIssues = prometheus.NewDesc(
		"total_availability_issues",
		"Number of total availability issues",
		nil, nil,
	)
	e.metrics.memoryAvailabilityIssues = prometheus.NewDesc(
		"memory_availability_issues",
		"Number of memory availability issues",
		nil, nil,
	)
	e.metrics.connectivityIssues = prometheus.NewDesc(
		"connectivity_issues",
		"Number of connectivity issues",
		nil, nil,
	)
	e.metrics.otherIssues = prometheus.NewDesc(
		"other_issues",
		"Number of unspecified issues",
		nil, nil,
	)

	// Table sizes
	e.metrics.tableSize = prometheus.NewDesc(
		"table_size",
		"RethinkDB table size in MB",
		[]string{"db", "table"}, nil,
	)

The changes I made are here, for reference: https://github.com/pauljwil/rethinkdb-prometheus-exporter/tree/master/exporter