Use ProxyVariableSubstitutionService for variable substitution in alerts

Merged Reuben Pereira requested to merge 220144-substitute-variables-in-alerts into master

What does this MR do?

Use the Prometheus::ProxyVariableSubstitutionService service to substitute variables in alert queries. This is the same service that is used to substitute variables in metrics. This puts alerts on par with metrics. Using the same service ensures that any change in variable substitution logic that affects alerts will be caught by alert specs.

Issue: #220144 (closed)

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Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

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