feat(GlSingleStat): support string and number for value prop

What does this MR do?

feat(GlSingleStat): allow unit prop to accept s... (!1983 - merged) added support for setting the unit prop to a string or number, but I think this was a mistake from the originating issue, it makes more sense for the value prop to support a string / number, the unit should most likely always be a string.

The original context for this change is this comment in the MR analytics utils which indicates that it should be the value prop that is changed.

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