[SDK] Add Some Checks for metrics Field in report_metrics() Interface

[Electronic-Waste]

What you would like to be added?

We decide to add some conditional checks in report_metrics() to ensure it works as expected when encounting some tricky corner case and enhance its robustness.

Why is this needed?

In the AutoML and Training WG Community Call, @tenzen-y asked what would happen if we pushed the same metrics twice in a single container. However, the current version of the report_metrics() API does not process this corner case.

Thus, we decided to raise an issue discussing the possible corner case that report_metrics() might meet with. Everyone is welcome to put your valuable insights and suggestions here!

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[andreyvelich]

/remove-label lifecycle/needs-triage
/area sdk
/good-first-issue

[google-oss-prow]

@andreyvelich:
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In response to this:

/remove-label lifecycle/needs-triage
/area sdk
/good-first-issue

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[Electronic-Waste]

I'd like to work on this since this is related to my GSoC project: #2340

/assign

[Electronic-Waste]

After investigating this issue, I find that:

1. If we pass two same keys in metrics: the latter one will overwrite the former one.

2. If we call report_metrics() twice and report the same metrics: they won't interfere with each other since they have different timestamp and GetObservationLog will retrieve both of them to the return value.

katib/pkg/db/v1beta1/mysql/mysql.go

Lines 134 to 135 in bc09cfd

	rows, err := d.db.Query("SELECT time, metric_name, value FROM observation_logs WHERE trial_name = ?"+qstr+" ORDER BY time",
	qfield...)