Alerting: Emit warning when creating or updating unusually large groups (#82279)

* Add config for limit of rules per rule group

* Warn when editing big groups through normal API

* Warn on prov api writes for groups

* Wire up comp root, tests

* Also add warning to state manager warm

* Drop unnecessary conversion

pkg/services/ngalert/api/api_ruler.go

@@ -251,6 +251,10 @@ func (srv RulerSrv) RoutePostNameRulesConfig(c *contextmodel.ReqContext, ruleGro
 		return toNamespaceErrorResponse(err)
 	}
 
+	if err := srv.checkGroupLimits(ruleGroupConfig); err != nil {
+		return ErrResp(http.StatusBadRequest, err, "")
+	}
+
 	rules, err := validateRuleGroup(&ruleGroupConfig, c.SignedInUser.GetOrgID(), namespace, srv.cfg)
 	if err != nil {
 		return ErrResp(http.StatusBadRequest, err, "")
@@ -265,6 +269,18 @@ func (srv RulerSrv) RoutePostNameRulesConfig(c *contextmodel.ReqContext, ruleGro
 	return srv.updateAlertRulesInGroup(c, groupKey, rules)
 }
 
+func (srv RulerSrv) checkGroupLimits(group apimodels.PostableRuleGroupConfig) error {
+	if srv.cfg.RulesPerRuleGroupLimit > 0 && int64(len(group.Rules)) > srv.cfg.RulesPerRuleGroupLimit {
+		srv.log.Warn("Large rule group was edited. Large groups are discouraged and may be rejected in the future.",
+			"limit", srv.cfg.RulesPerRuleGroupLimit,
+			"actual", len(group.Rules),
+			"group", group.Name,
+		)
+	}
+
+	return nil
+}
+
 // updateAlertRulesInGroup calculates changes (rules to add,update,delete), verifies that the user is authorized to do the calculated changes and updates database.
 // All operations are performed in a single transaction
 func (srv RulerSrv) updateAlertRulesInGroup(c *contextmodel.ReqContext, groupKey ngmodels.AlertRuleGroupKey, rules []*ngmodels.AlertRuleWithOptionals) response.Response {