fix(otelgorm): Make metric reporting work with prepared statements

Previously we only initialized metrics reporting by calling
otelsql.ReportDBStatsMetrics if the gorm.DB.ConnPool interface
pointer we received happened to be a *sql.DB. This was often
the case, but not if prepared statements were enabled (and
possibly in other circumstances too). This caused metrics
reporting to be silently disabled. We fix this by calling
gorm.DB.DB whose sole purpose is to extract the underlying
*sql.DB.

otelgorm/otelgorm.go

@@ -55,8 +55,8 @@ type gormRegister interface {
 
 func (p otelPlugin) Initialize(db *gorm.DB) (err error) {
 	if !p.excludeMetrics {
-		if db, ok := db.ConnPool.(*sql.DB); ok {
-			otelsql.ReportDBStatsMetrics(db)
+		if sqlDB, err := db.DB(); err == nil {
+			otelsql.ReportDBStatsMetrics(sqlDB)
 		}
 	}