Add support for Prometheus api prefix (#341)

Signed-off-by: Lauri Nevala <lauri.nevala@gmail.com>

src/main/cluster.ts

@@ -46,6 +46,7 @@ export type ClusterPreferences = {
     namespace: string;
     service: string;
     port: number;
+    prefix: string;
   };
   prometheusProvider?: {
     type: string;
@@ -153,6 +154,13 @@ export class Cluster implements ClusterInfo {
     this.save()
   }
 
+  public getPrometheusApiPrefix() {
+    if (!this.preferences.prometheus?.prefix) {
+      return ""
+    }
+    return this.preferences.prometheus.prefix
+  }
+
   public save() {
     clusterStore.storeCluster(this)
   }