Support server name configuration

[davidkemper]

Recently the nats-server added support for a more human-readable server name.

It would be nice to add support for it in the nats-operator.

As it stands now, the server identifier (e.g., NAICD2C2WZ62VH7QELNX6LLI6BBY34SL3MVD3FCJVMJCSOOBOZ7YZ7N3) appears in monitoring stacks like Prometheus. While unique, it is not friendly.

A possible implementation might chose an operator DSL attribute to control the name prefix of the server. For example, <prefix>-<server-number> ==> mycluster-1 for the first server. The prefix could default to the nats-cluster metadata name. Note that using just the nats-cluster metadata name might result in naming collisions when consolidating monitoring for multiple clusters, so allowing an explicit prefix would be "nice".

The nats-operator currently uses a single configuration secret that it maps to each server pod. I believe this makes the solution non-trivial.

[wallyqs]

The problem with the shared config is that since JSON is used to marshal the config, then cannot use an environment variable to set the server name since it will be using quotes. The options are then:

• Improve the config marshalling of the nats operator so that can have fields that are environment variables which are unquoted
• Make nats-servers be able to set a name via the command line instead of via the config where $POD_NAME would be used
• Change nats-server to use some environment variable as the server_name instead of config

Reference example of how this works with statefulsets: https://gist.github.com/wallyqs/025d3ceffe2d40cec7ce02a97ae76617#file-server-name-yml-L8