What is the default username and password for Grafana login page?

By looking up the docs we can find that the magic combo is admin as username and admin as password.

However if you changed some configuration file you should be able to find it there. The default config file can be found here: $WORKING_DIR/conf/defaults.ini and can be overridden using the --config parameter

The item in the config you're looking for should be in the section:

[security]
admin_user = admin
admin_password = admin

If you are using Prometheus Operator then user/pass is:

user: admin
pass: prom-operator

Install prometheus-operator using helm:

#helm3
helm repo add stable https://kubernetes-charts.storage.googleapis.com

helm install my-prometheus-operator stable/prometheus-operator