add authn label

[hzhao-wish]

Test the non-authn connection locally.

mongoproxy_plugins_mongo_command_duration_seconds{authn="false",collection="test",command="find",db="test",readpref="primary",quantile="0.99"} 0.003038701
mongoproxy_plugins_mongo_command_duration_seconds{authn="false",collection="test",command="find",db="test",readpref="primary",quantile="1"} 0.003038701
mongoproxy_plugins_mongo_command_duration_seconds_sum{authn="false",collection="test",command="find",db="test",readpref="primary"} 0.003038701
mongoproxy_plugins_mongo_command_duration_seconds_count{authn="false",collection="test",command="find",db="test",readpref="primary"} 1

Hard to test authn connection.