Reuse connections on sql module (elastic#16001)

Keep a connection open with the database instead of opening
and closing connections on each fetch. this is consistent with
other specific SQL modules we have.

(cherry picked from commit 583a89c)

CHANGELOG.next.asciidoc

@@ -200,6 +200,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add support for Unix socket in Memcached metricbeat module. {issue}13685[13685] {pull}15822[15822]
 - Make the `system/cpu` metricset collect normalized CPU metrics by default. {issue}15618[15618] {pull}15729[15729]
 - Add support for processors in light modules. {issue}14740[14740] {pull}15923[15923]
+- Reuse connections in SQL module. {pull}16001[16001]
 
 *Packetbeat*
 

x-pack/metricbeat/module/sql/query/query.go

@@ -5,6 +5,7 @@
 package query
 
 import (
+	"context"
 	"fmt"
 	"strconv"
 	"strings"
@@ -43,6 +44,8 @@ type MetricSet struct {
 	Driver         string
 	Query          string
 	ResponseFormat string
+
+	db *sqlx.DB
 }
 
 // New creates a new instance of the MetricSet. New is responsible for unpacking
@@ -77,19 +80,13 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 // of an error set the Error field of mb.Event or simply call report.Error().
 // It calls m.fetchTableMode() or m.fetchVariableMode() depending on the response
 // format of the query.
-func (m *MetricSet) Fetch(report mb.ReporterV2) error {
-	db, err := sqlx.Open(m.Driver, m.HostData().URI)
+func (m *MetricSet) Fetch(ctx context.Context, report mb.ReporterV2) error {
+	db, err := m.DB()
 	if err != nil {
 		return errors.Wrap(err, "error opening connection")
 	}
-	defer db.Close()
-
-	err = db.Ping()
-	if err != nil {
-		return errors.Wrap(err, "error testing connection")
-	}
 
-	rows, err := db.Queryx(m.Query)
+	rows, err := db.QueryxContext(ctx, m.Query)
 	if err != nil {
 		return errors.Wrap(err, "error executing query")
 	}
@@ -102,6 +99,23 @@ func (m *MetricSet) Fetch(report mb.ReporterV2) error {
 	return m.fetchVariableMode(rows, report)
 }
 
+// DB gets a client ready to query the database
+func (m *MetricSet) DB() (*sqlx.DB, error) {
+	if m.db == nil {
+		db, err := sqlx.Open(m.Driver, m.HostData().URI)
+		if err != nil {
+			return nil, errors.Wrap(err, "opening connection")
+		}
+		err = db.Ping()
+		if err != nil {
+			return nil, errors.Wrap(err, "testing connection")
+		}
+
+		m.db = db
+	}
+	return m.db, nil
+}
+
 // fetchTableMode scan the rows and publishes the event for querys that return the response in a table format.
 func (m *MetricSet) fetchTableMode(rows *sqlx.Rows, report mb.ReporterV2) error {
 
@@ -229,3 +243,11 @@ func getValue(pval *interface{}) string {
 		return fmt.Sprint(v)
 	}
 }
+
+// Close closes the connection pool releasing its resources
+func (m *MetricSet) Close() error {
+	if m.db == nil {
+		return nil
+	}
+	return errors.Wrap(m.db.Close(), "closing connection")
+}