web api: handle alert with Infinity/NaN values (#5582)

* web/api/v1: alert value as string in alert/rules endpoints

Signed-off-by: Alexander Saltykov <alexander-s@yandex-team.ru>

docs/querying/api.md

@@ -465,7 +465,7 @@ $ curl http://localhost:9090/api/v1/rules
                                     "severity": "page"
                                 },
                                 "state": "firing",
-                                "value": 1
+                                "value": "1e+00"
                             }
                         ],
                         "annotations": {
@@ -522,7 +522,7 @@ $ curl http://localhost:9090/api/v1/alerts
                     "alertname": "my-alert"
                 },
                 "state": "firing",
-                "value": 1
+                "value": "1e+00"
             }
         ]
     },

web/api/v1/api.go

@@ -694,7 +694,7 @@ type Alert struct {
 	Annotations labels.Labels `json:"annotations"`
 	State       string        `json:"state"`
 	ActiveAt    *time.Time    `json:"activeAt,omitempty"`
-	Value       float64       `json:"value"`
+	Value       string        `json:"value"`
 }
 
 func (api *API) alerts(r *http.Request) apiFuncResult {
@@ -721,7 +721,7 @@ func rulesAlertsToAPIAlerts(rulesAlerts []*rules.Alert) []*Alert {
 			Annotations: ruleAlert.Annotations,
 			State:       ruleAlert.State.String(),
 			ActiveAt:    &ruleAlert.ActiveAt,
-			Value:       ruleAlert.Value,
+			Value:       strconv.FormatFloat(ruleAlert.Value, 'e', -1, 64),
 		}
 	}