expose the running version

[woodsaj]

When looking into issues, the first question is always "what version are you using".
To make it clear, we should expose the version via

• the UI
• emit a metric with the version information.

[Dieterbe]

i'm surprised that this is currently considered not easy enough.

• you can get the version by running carbon-relay-ng version. maybe people didn't know about this, so i added it to the usage/help output in #324
• otherwise you can get it by looking which package you installed, which docker image you're running etc.

that said, we can do both of the things requested. though i'm not sure if those approaches are more straightforward than what we already have.

[woodsaj]

In dockerized environments, running commands or looking at package versions inside a container is not always an option.