Make Key Metric tooltips more insightful

## Feature Description

This slack discussion summarised this issue: https://10up.slack.com/archives/C01CA828ZPZ/p1696606229758509

Contents of the slack discussion:
We had a conversation on our sync on 5 Oct regarding aligning our tooltips that popup on the KMW tiles and the descriptions within the Metrics Selection panel. We came up with the following options:

1. Make the tooltips and descriptions exactly the same. Currently, there are only minor differences for the old KMW tiles since the tooltips were written by you separately from the descriptions that were written in Figma for the Metrics panel. If there are only minor differences, it makes more sense to make them exactly the same (and we can clean up the code here too).

2. Another suggestion we had after seeing @sigal-teller’s and your great work on the audience segmentation Figma was the “Insights message banner” which explains “why” a particular metric is useful, i.e. the New Visitors shows that your content is helpful…. and the returning visitors shows you how loyal your visitors are returning because your website has information that is xyz.. I really liked these little insights and think that we could draft messages like these in our tooltips. Currently, our tooltips are quite dry and simply explain the title using some synonyms. If we can draft the reasons why a metric is useful, then we could potentially change the content of the tooltips and use the current tooltip content for descriptions in the Metrics panel. We do have “some” more space in a tooltip compared to in the metric panel where we’ve got rid of the accordion and have many tiles to show now.

Keeping the tooltip and descriptions the way they are right now is pointless, since we either align them fully or go with option 2 and draft more meaningful tooltips.

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@marrrmarrr to come up with some more insightful tooltips or decide to standardise them with the descriptions.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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