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
```

einterfaces/metrics.go

@@ -4,12 +4,15 @@
 package einterfaces
 
 import (
+	"database/sql"
+
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 type MetricsInterface interface {
 	Register()
+	RegisterDBCollector(db *sql.DB, name string)
 
 	IncrementPostCreate()
 	IncrementWebhookPost()

einterfaces/mocks/MetricsInterface.go

@@ -9,6 +9,8 @@ import (
 	mock "github.com/stretchr/testify/mock"
 
 	model "github.com/mattermost/mattermost-server/v6/model"
+
+	sql "database/sql"
 )
 
 // MetricsInterface is an autogenerated mock type for the MetricsInterface type
@@ -302,6 +304,11 @@ func (_m *MetricsInterface) Register() {
 	_m.Called()
 }
 
+// RegisterDBCollector provides a mock function with given fields: db, name
+func (_m *MetricsInterface) RegisterDBCollector(db *sql.DB, name string) {
+	_m.Called(db, name)
+}
+
 // SetReplicaLagAbsolute provides a mock function with given fields: node, value
 func (_m *MetricsInterface) SetReplicaLagAbsolute(node string, value float64) {
 	_m.Called(node, value)