Add how-to for using Pebble's metrics endpoint in charm

[benhoyt]

After Ops has support for identities (#1594), you'll be able to add a "metrics" identity to allow Prometheus or whatever tool to hit Pebble's metrics endpoint. But we should write a how-to explaining the various steps involved: generating a password, using Container.replace_identities, adding a secret, and setting up Prometheus with the secret.