Show metrics icon in sidebar if prometheus-addr is specified

ui/src/App.tsx

@@ -22,6 +22,7 @@ import LayersIcon from "@material-ui/icons/Layers";
 import SettingsIcon from "@material-ui/icons/Settings";
 import ScheduleIcon from "@material-ui/icons/Schedule";
 import FeedbackIcon from "@material-ui/icons/Feedback";
+import TimelineIcon from "@material-ui/icons/Timeline";
 import DoubleArrowIcon from "@material-ui/icons/DoubleArrow";
 import CloseIcon from "@material-ui/icons/Close";
 import { AppState } from "./store";
@@ -244,6 +245,13 @@ function App(props: ConnectedProps<typeof connector>) {
                       primary="Redis"
                       icon={<LayersIcon />}
                     />
+                    {window.PROMETHEUS_SERVER_ADDRESS && (
+                      <ListItemLink
+                        to="/TODO"
+                        primary="Metrics"
+                        icon={<TimelineIcon />}
+                      />
+                    )}
                   </div>
                 </List>
                 <List>

ui/src/global.d.ts

@@ -2,4 +2,8 @@ interface Window {
   // Root URL path for asynqmon app.
   // ROOT_PATH should not have the tailing slash.
   ROOT_PATH: string;
-}
+
+  // Prometheus server address to query time series data.
+  // This field is set to empty string by default. Use this field only if it's set.
+  PROMETHEUS_SERVER_ADDRESS: string;
+}