feat(database/metadata): implement mysql method createEntity

Implementation of createEntity is pretty different between postgres
and mysql, reimplement it for mysql.

internal/database/metadata/mysql/entity.go

@@ -0,0 +1,24 @@
+package mysql
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/go-sql-driver/mysql"
+	"github.com/oom-ai/oomstore/internal/database/metadata"
+)
+
+func createEntity(ctx context.Context, sqlxCtx metadata.SqlxContext, opt metadata.CreateEntityOpt) (int, error) {
+	query := "INSERT INTO entity(name, length, description) VALUES(?, ?, ?)"
+	res, err := sqlxCtx.ExecContext(ctx, query, opt.EntityName, opt.Length, opt.Description)
+	if er, ok := err.(*mysql.MySQLError); ok {
+		if er.Number == ER_DUP_ENTRY {
+			return 0, fmt.Errorf("entity %s already exists", opt.EntityName)
+		}
+	}
+	entityID, err := res.LastInsertId()
+	if err != nil {
+		return 0, err
+	}
+	return int(entityID), err
+}