Configure proxy for XCMetrics backend

Add configuration for proxying requests to '/xcmetrics' to
local instance of a XCMetrics backend (http://localhost:8080/v1)

Signed-off-by: Niklas Granander <ngranander@spotify.com>

app-config.yaml

@@ -92,6 +92,9 @@ proxy:
     headers:
       Authorization: ${ILERT_AUTH_HEADER}
 
+  '/xcmetrics':
+    target: http://127.0.0.1:8080/v1
+
 organization:
   name: My Company