Expose when Rule alert labels are being overwritten by its --label(s)

[filippog]

Is your proposal related to a problem?

We run thanos rule to great success! We do also set a few --label as recommended (e.g. replica) by https://thanos.io/tip/components/rule.md/#external-labels . With multiple folks writing alerts it has happened in the past that alert labels ended up clashing with rule's external labels, effectively losing the alert's labels.

Describe the solution you'd like

I think it would be great to be able to at least have some sort of signal when alert labels are being overwritten by rule external labels, for example in a dedicated metric.