pulsar-proxy: fix env var enable unauth metrics

[acjohnson]

The bin/apply-config-from-env.py script will not apply the authenticateMetricsEndpoint config value unless it's already set in conf/proxy.conf, which unfortunately in the datastax/lunastreaming-all:2.10_5.1 image, it is not set...

It might be preferable in this case to have the example value to include the prefix to make proxy metrics a bit easier to enable.