Fix wrong path in metric migration

[peterjan]

Recently had to add a metric migration and found that we don't properly build the path in performMigration, it always uses main:

// build path
var path string
if isSQLite(db) {
    path = fmt.Sprintf("migrations/sqlite/main/migration_" + name + ".sql")
} else {
    path = fmt.Sprintf("migrations/mysql/main/migration_" + name + ".sql")
}

Tested it with a dummy metrics/main migration on my MySQL node.