Ability to override prometheus rule expressions

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Hello, we have a use case where we install this helm chart on a number of clusters but all the prometheus rules are observed by a single alert manager installation. Since we install this helm chart within the scope of a cluster, it would be helpful if we had the option to override the prometheus rule expressions to include our own filters such that we receive a single alert instead of n alerts, with n being the number of clusters.

I'm happy to file a pull request.