Prometheus: Interpolate vars in adhoc filters request (#94087)

* Interpolate vars in adhoc filters request

* interpolate variables on BE

---------

Co-authored-by: Kyle Brandt <kyle@grafana.com>

pkg/promlib/models/query.go

@@ -207,7 +207,7 @@ func Parse(span trace.Span, query backend.DataQuery, dsScrapeInterval string, in
 
 	// Interpolate variables in expr
 	timeRange := query.TimeRange.To.Sub(query.TimeRange.From)
-	expr := interpolateVariables(
+	expr := InterpolateVariables(
 		model.Expr,
 		query.Interval,
 		calculatedStep,
@@ -365,14 +365,14 @@ func calculateRateInterval(
 	return rateInterval
 }
 
-// interpolateVariables interpolates built-in variables
+// InterpolateVariables interpolates built-in variables
 // expr                         PromQL query
 // queryInterval                Requested interval in milliseconds. This value may be overridden by MinStep in query options
 // calculatedStep               Calculated final step value. It was calculated in calculatePrometheusInterval
 // requestedMinStep             Requested minimum step value. QueryModel.interval
 // dsScrapeInterval             Data source scrape interval in the config
 // timeRange                    Requested time range for query
-func interpolateVariables(
+func InterpolateVariables(
 	expr string,
 	queryInterval time.Duration,
 	calculatedStep time.Duration,