Subject: Communication Issue Between Viz and External Prometheus

Problem: In the documentation explaining the usage of an external Prometheus, the protocol is missing in provided example. This is causing Viz to be unable to establish communication with the external Prometheus, resulting in a "unsupported protocol" error message in the logs.

Solution: added protocol and basic auth to prometheus url

Fixes linkerd#1658

Signed-off-by: JadKHaddad <jadkhaddad@gmail.com>

linkerd.io/content/2.13/tasks/external-prometheus.md

@@ -140,15 +140,15 @@ on the Prometheus instance to power the dashboard and CLI.
 
 The `prometheusUrl` field gives you a single place through
 which all these components can be configured to an external Prometheus URL.
-This is allowed both through the CLI and Helm.
+This is allowed both through the CLI and Helm. If the external Prometheus is secured with basic auth, you can include the credentials in the URL as well.
 
 ### CLI
 
 This can be done by passing a file with the above field to the `values` flag,
 which is available through `linkerd viz install` command.
 
 ```yaml
-prometheusUrl: existing-prometheus.xyz:9090
+prometheusUrl: http://username:password@existing-prometheus.namespace:9090
 ```
 
 Once applied, this configuration is not persistent across installs.