I would not personally build a TSDB atop MySQL, though I worked at a place that did, and it worked OK. I don't remember their schema, though.

If I had to, I'd probably do something like this (haven't tested it beyond validating that it creates):

    mysql> SHOW CREATE TABLE ts\G
    *************************** 1. row ***************************
           Table: ts
    Create Table: CREATE TABLE `ts` (
      `metric_id` smallint unsigned NOT NULL,
      `ts` datetime NOT NULL,
      PRIMARY KEY (`metric_id`,`ts`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
    /*!50100 PARTITION BY RANGE (dayofmonth(`ts`))
    SUBPARTITION BY HASH (`metric_id`)
    SUBPARTITIONS 3
    (PARTITION p1 VALUES LESS THAN (2) ENGINE = InnoDB,
     PARTITION p2 VALUES LESS THAN (3) ENGINE = InnoDB,
     PARTITION p3 VALUES LESS THAN MAXVALUE ENGINE = InnoDB) */
    1 row in set (0.012 sec)

Obviously there would be more partitions for different days (or whatever other date chunk you wanted – months, weeks, etc.), and the sub partitions number would depend on how many metrics you were tracking. You could also simplify this at the expense of more tables by having a table per metric.