Loki: Set Query limits context header (#114112)

* feat(lokiQueryLimitsContext): send full query range & expr on initial split requests

pkg/tsdb/loki/parse_query.go

@@ -156,6 +156,8 @@ func parseQuery(queryContext *backend.QueryDataRequest, logqlScopesEnabled bool)
 
 		expr := interpolateVariables(model.Expr, interval, timeRange, queryType, step)
 
+		limitsConfig := generateLimitsConfig(model, interval, timeRange, queryType, step)
+
 		direction, err := parseDirection(model.Direction)
 		if err != nil {
 			return nil, err
@@ -192,8 +194,21 @@ func parseQuery(queryContext *backend.QueryDataRequest, logqlScopesEnabled bool)
 			RefID:               query.RefID,
 			SupportingQueryType: supportingQueryType,
 			Scopes:              model.Scopes,
+			LimitsContext:       limitsConfig,
 		})
 	}
 
 	return qs, nil
 }
+
+func generateLimitsConfig(model *QueryJSONModel, interval time.Duration, timeRange time.Duration, queryType QueryType, step time.Duration) LimitsContext {
+	var limitsConfig LimitsContext
+	// Only supply limits context config if we have expression, and from and to
+	if model.LimitsContext != nil && model.LimitsContext.Expr != "" && model.LimitsContext.From > 0 && model.LimitsContext.To > 0 {
+		// If a limits expression was provided, interpolate it and parse the time range
+		limitsConfig.Expr = interpolateVariables(model.LimitsContext.Expr, interval, timeRange, queryType, step)
+		limitsConfig.From = time.UnixMilli(model.LimitsContext.From)
+		limitsConfig.To = time.UnixMilli(model.LimitsContext.To)
+	}
+	return limitsConfig
+}