[file-metricscollector]: metrics should be formatted as string

[axel7083]

The following line, will try to cast a string any value provided.

https://github.com/kubeflow/katib/blob/37b237f56091a36db35cf4a4148847d1da572278/pkg/metricscollector/v1beta1/file-metricscollector/file-metricscollector.go#L139

This will make fail any other value (float, double etc.). If this is the expected behavior, it should probably be documented. The example available in the katib documentation shows as example:

{"epoch": 0, "foo": “bar", “fizz": “buzz", "timestamp": 1638422847.28721…}
{"epoch": 1, "foo": “bar", “fizz": “buzz", "timestamp": 1638422847.287801…}
{"epoch": 2, "foo": “bar", “fizz": “buzz", "timestamp": "2021-12-02T14:27:50.000035161+09:00"…}
{"epoch": 3, "foo": “bar", “fizz": “buzz", "timestamp": "2021-12-02T14:27:50.000037459+09:00"…}

Does not really shows that the value should be between ".

[tenzen-y]

@axel7083 Thanks for creating this issue. I'm a creator of the feature. Casting to string is intended since I assumed to output the log by hypertune, which outputs string value.

https://github.com/GoogleCloudPlatform/cloudml-hypertune/blob/8e3530e1c4926ac64cf28330d1104b838e07a468/hypertune/hypertune.py#L64-L74

Maybe, it is hard to support non-string values since we can not know which type the value is using in advance.

So I agree with mentioning the limitation in the doc.

cc: @kubeflow/wg-automl-leads

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