MM-39215: Add DBStatsCollector to metrics (#21957)

https://mattermost.atlassian.net/browse/MM-39215

```release-note
We now have Grafana metrics for DB connection metrics.
They are:
max_open_connections
open_connections
in_use_connections
idle_connections
wait_count_total
wait_duration_seconds_total
max_idle_closed_total
max_idle_time_closed_total
max_lifetime_closed_total
```

store/sqlstore/store.go

@@ -301,6 +301,9 @@ func (ss *SqlStore) initConnection() {
 	if ss.DriverName() == model.DatabaseDriverMysql {
 		ss.masterX.MapperFunc(noOpMapper)
 	}
+	if ss.metrics != nil {
+		ss.metrics.RegisterDBCollector(ss.masterX.DB.DB, "master")
+	}
 
 	if len(ss.settings.DataSourceReplicas) > 0 {
 		ss.ReplicaXs = make([]*sqlxDBWrapper, len(ss.settings.DataSourceReplicas))
@@ -312,6 +315,9 @@ func (ss *SqlStore) initConnection() {
 			if ss.DriverName() == model.DatabaseDriverMysql {
 				ss.ReplicaXs[i].MapperFunc(noOpMapper)
 			}
+			if ss.metrics != nil {
+				ss.metrics.RegisterDBCollector(ss.ReplicaXs[i].DB.DB, "replica-"+strconv.Itoa(i))
+			}
 		}
 	}
 
@@ -325,6 +331,9 @@ func (ss *SqlStore) initConnection() {
 			if ss.DriverName() == model.DatabaseDriverMysql {
 				ss.searchReplicaXs[i].MapperFunc(noOpMapper)
 			}
+			if ss.metrics != nil {
+				ss.metrics.RegisterDBCollector(ss.searchReplicaXs[i].DB.DB, "searchreplica-"+strconv.Itoa(i))
+			}
 		}
 	}
 

store/sqlstore/store_test.go

@@ -24,6 +24,7 @@ import (
 	"github.com/mattermost/mattermost-server/v6/db"
 	"github.com/mattermost/mattermost-server/v6/einterfaces/mocks"
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock"
 	"github.com/mattermost/mattermost-server/v6/store"
 	"github.com/mattermost/mattermost-server/v6/store/searchtest"
 	"github.com/mattermost/mattermost-server/v6/store/storetest"
@@ -731,6 +732,7 @@ func TestReplicaLagQuery(t *testing.T) {
 		defer mockMetrics.AssertExpectations(t)
 		mockMetrics.On("SetReplicaLagAbsolute", tableName, float64(1))
 		mockMetrics.On("SetReplicaLagTime", tableName, float64(1))
+		mockMetrics.On("RegisterDBCollector", mock.AnythingOfType("*sql.DB"), "master")
 
 		store := &SqlStore{
 			rrCounter: 0,