Alerting: send alerts to external, internal, or both alertmanagers (#40341)

* (WIP) send alerts to external, internal, or both alertmanagers

* Modify admin configuration endpoint, update swagger docs

* Integration test for admin config updated

* Code review changes

* Fix alertmanagers choice not changing bug, add unit test

* Add AlertmanagersChoice as enum in swagger, code review changes

* Fix API and tests errors

* Change enum from int to string, use 'SendAlertsTo' instead of 'AlertmanagerChoice' where necessary

* Fix tests to reflect last changes

* Keep senders running when alerts are handled just internally

* Check if any external AM has been discovered before sending alerts, update tests

* remove duplicate data from logs

* update comment

* represent alertmanagers choice as an int instead of a string

* default alertmanagers choice to all alertmanagers, test cases

* update definitions and generate spec

pkg/services/ngalert/models/admin_configuration_test.go

@@ -1,6 +1,7 @@
 package models
 
 import (
+	"errors"
 	"fmt"
 	"testing"
 
@@ -56,3 +57,56 @@ func TestAdminConfiguration_Validate(t *testing.T) {
 		})
 	}
 }
+
+func TestStringToAlertmanagersChoice(t *testing.T) {
+	tests := []struct {
+		name                string
+		str                 string
+		alertmanagersChoice AlertmanagersChoice
+		err                 error
+	}{
+		{
+			"all alertmanagers",
+			"all",
+			AllAlertmanagers,
+			nil,
+		},
+		{
+			"internal alertmanager",
+			"internal",
+			InternalAlertmanager,
+			nil,
+		},
+		{
+			"external alertmanagers",
+			"external",
+			ExternalAlertmanagers,
+			nil,
+		},
+		{
+			"empty string value",
+			"",
+			AllAlertmanagers,
+			nil,
+		},
+		{
+			"invalid string",
+			"invalid",
+			0,
+			errors.New("invalid alertmanager choice"),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(tt *testing.T) {
+			amc, err := StringToAlertmanagersChoice(test.str)
+			if test.err != nil {
+				require.EqualError(tt, err, test.err.Error())
+			} else {
+				require.NoError(tt, err)
+			}
+
+			require.Equal(tt, amc, test.alertmanagersChoice)
+		})
+	}
+}