Added clarification and fix the Metrics doc

[khrm]

Clarified why pipelinerun level metrics aren't recommended. Also, fixed the documentation for a type.

Changes

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Release Notes

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[khrm]

/kind documentation

[Yongxuanzhang]

/lgtm

[khrm]

/assign @lbernick

[khrm]

/assign @vdemeester

[vdemeester]

@khrm can you take @afrittoli's comment into account ? 🙏🏼

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[khrm]

It's merged. I will raise some metrics pr soon, I would then take @afrittoli's comment into account.