Add performance_schema.status_by_host gauge

This commit aims at adding a gauge measuring the contents of the
`performance_schema.status_by_host` table

Gauge name is `mysql_perf_schema_status_by_host`,
with labels `host` and `variable_name`

collector/perf_schema_status_by_host.go

@@ -0,0 +1,87 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Scrape `performance_schema.status_by_host` column information.
+
+package collector
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/go-kit/kit/log"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const perfStatusByHostQuery = `
+	SELECT HOST, VARIABLE_NAME, VARIABLE_VALUE
+	FROM performance_schema.status_by_host
+	WHERE HOST IS NOT NULL;
+	`
+
+// Metric descriptors.
+var (
+	performanceSchemaStatusByHostDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, performanceSchema, "status_by_host"),
+		"The status variable value for a given host.",
+		[]string{"host", "variable_name"}, nil,
+	)
+)
+
+// ScrapePerfStatusByHost collects status_by_host table information.
+type ScrapePerfStatusByHost struct{}
+
+// Name of the Scraper. Should be unique.
+func (ScrapePerfStatusByHost) Name() string {
+	return "perf_schema.status_by_host"
+}
+
+// Help describes the role of the Scraper.
+func (ScrapePerfStatusByHost) Help() string {
+	return "Collect status_by_host gauges in performance_schema"
+}
+
+// Version of MySQL from which scraper is available.
+func (ScrapePerfStatusByHost) Version() float64 {
+	return 5.7
+}
+
+// Scrape collects data from database connection and sends it over channel as prometheus metric.
+func (ScrapePerfStatusByHost) Scrape(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric, logger log.Logger) error {
+	statusByHostRows, err := db.QueryContext(ctx, perfStatusByHostQuery)
+	if err != nil {
+		return err
+	}
+	defer statusByHostRows.Close()
+
+	var (
+		host, variable_name string
+		value               float64
+	)
+
+	for statusByHostRows.Next() {
+		if err := statusByHostRows.Scan(
+			&host, &variable_name, &value,
+		); err != nil {
+			return err
+		}
+		ch <- prometheus.MustNewConstMetric(
+			performanceSchemaStatusByHostDesc, prometheus.GaugeValue, value,
+			host, variable_name,
+		)
+	}
+	return nil
+}
+
+// check interface
+var _ Scraper = ScrapePerfStatusByHost{}

mysqld_exporter.go

@@ -86,6 +86,7 @@ var scrapers = map[collector.Scraper]bool{
 	collector.ScrapePerfReplicationGroupMembers{}:         false,
 	collector.ScrapePerfReplicationGroupMemberStats{}:     false,
 	collector.ScrapePerfReplicationApplierStatsByWorker{}: false,
+	collector.ScrapePerfStatusByHost{}:                    false,
 	collector.ScrapeUserStat{}:                            false,
 	collector.ScrapeClientStat{}:                          false,
 	collector.ScrapeTableStat{}:                           false,