Alerting: Introduces /api/v1/ngalert/alertmanagers to expose discovered and dropped Alertmanager(s) (#37632)

* Alerting: Expose discovered and dropped Alertmanagers

Exposes the API for discovered and dropped Alertmanagers.

* make admin config poll interval configurable

* update after rebase

* wordsmith

* More wordsmithing

* change name of the config

* settings package too

pkg/services/ngalert/api/tooling/post.json

@@ -57,12 +57,8 @@
    "type": "object",
    "x-go-package": "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
   },
-  "AlertGroup": {
-   "$ref": "#/definitions/alertGroup"
-  },
-  "AlertGroups": {
-   "$ref": "#/definitions/alertGroups"
-  },
+  "AlertGroup": {},
+  "AlertGroups": {},
   "AlertInstancesResponse": {
    "properties": {
     "instances": {
@@ -81,6 +77,38 @@
    "type": "object",
    "x-go-package": "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
   },
+  "AlertManager": {
+   "properties": {
+    "url": {
+     "type": "string",
+     "x-go-name": "URL"
+    }
+   },
+   "title": "AlertManager models a configured Alert Manager.",
+   "type": "object",
+   "x-go-package": "github.com/prometheus/client_golang/api/prometheus/v1"
+  },
+  "AlertManagersResult": {
+   "properties": {
+    "activeAlertManagers": {
+     "items": {
+      "$ref": "#/definitions/AlertManager"
+     },
+     "type": "array",
+     "x-go-name": "Active"
+    },
+    "droppedAlertManagers": {
+     "items": {
+      "$ref": "#/definitions/AlertManager"
+     },
+     "type": "array",
+     "x-go-name": "Dropped"
+    }
+   },
+   "title": "AlertManagersResult contains the result from querying the alertmanagers endpoint.",
+   "type": "object",
+   "x-go-package": "github.com/prometheus/client_golang/api/prometheus/v1"
+  },
   "AlertQuery": {
    "properties": {
     "datasourceUid": {
@@ -477,12 +505,21 @@
   "Failure": {
    "$ref": "#/definitions/ResponseDetails"
   },
-  "GettableAlert": {
-   "$ref": "#/definitions/gettableAlert"
-  },
-  "GettableAlerts": {
-   "$ref": "#/definitions/gettableAlerts"
+  "GettableAlert": {},
+  "GettableAlertmanagers": {
+   "properties": {
+    "data": {
+     "$ref": "#/definitions/AlertManagersResult"
+    },
+    "status": {
+     "type": "string",
+     "x-go-name": "Status"
+    }
+   },
+   "type": "object",
+   "x-go-package": "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
   },
+  "GettableAlerts": {},
   "GettableApiAlertingConfig": {
    "properties": {
     "global": {
@@ -786,8 +823,12 @@
    "type": "object",
    "x-go-package": "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
   },
-  "GettableSilence": {},
-  "GettableSilences": {},
+  "GettableSilence": {
+   "$ref": "#/definitions/gettableSilence"
+  },
+  "GettableSilences": {
+   "$ref": "#/definitions/gettableSilences"
+  },
   "GettableStatus": {
    "properties": {
     "cluster": {
@@ -2243,6 +2284,7 @@
    "x-go-package": "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
   },
   "URL": {
+   "description": "The general form represented is:\n\n[scheme:][//[userinfo@]host][/]path[?query][#fragment]\n\nURLs that do not start with a slash after the scheme are interpreted as:\n\nscheme:opaque[?query][#fragment]\n\nNote that the Path field is stored in decoded form: /%47%6f%2f becomes /Go/.\nA consequence is that it is impossible to tell which slashes in the Path were\nslashes in the raw URL and which were %2f. This distinction is rarely important,\nbut when it is, the code should use RawPath, an optional field which only gets\nset if the default encoding is different from Path.\n\nURL's String method uses the EscapedPath method to obtain the path. See the\nEscapedPath method for more details.",
    "properties": {
     "ForceQuery": {
      "type": "boolean"
@@ -2275,9 +2317,9 @@
      "$ref": "#/definitions/Userinfo"
     }
    },
-   "title": "URL is a custom URL type that allows validation at configuration load time.",
+   "title": "A URL represents a parsed URL (technically, a URI reference).",
    "type": "object",
-   "x-go-package": "github.com/prometheus/common/config"
+   "x-go-package": "net/url"
   },
   "Userinfo": {
    "description": "The Userinfo type is an immutable encapsulation of username and\npassword details for a URL. An existing Userinfo value is guaranteed\nto have a username set (potentially empty, as allowed by RFC 2396),\nand optionally a password.",
@@ -2449,7 +2491,7 @@
     "alerts": {
      "description": "alerts",
      "items": {
-      "$ref": "#/definitions/gettableAlert"
+      "$ref": "#/definitions/GettableAlert"
      },
      "type": "array",
      "x-go-name": "Alerts"
@@ -2729,7 +2771,7 @@
   "gettableSilences": {
    "description": "GettableSilences gettable silences",
    "items": {
-    "$ref": "#/definitions/GettableSilence"
+    "$ref": "#/definitions/gettableSilence"
    },
    "type": "array",
    "x-go-name": "GettableSilences",
@@ -3888,6 +3930,26 @@
     ]
    }
   },
+  "/api/v1/ngalert/alertmanagers": {
+   "get": {
+    "operationId": "RouteGetAlertmanagers",
+    "produces": [
+     "application/json"
+    ],
+    "responses": {
+     "200": {
+      "description": "GettableAlertmanagers",
+      "schema": {
+       "$ref": "#/definitions/GettableAlertmanagers"
+      }
+     }
+    },
+    "summary": "Get the discovered and dropped Alertmanagers of the user's organization based on the specified configuration.",
+    "tags": [
+     "configuration"
+    ]
+   }
+  },
   "/api/v1/receiver/test/{Recipient}": {
    "post": {
     "consumes": [