Add the tag lightstep.hostname

[zalando-andrewhowden-com]

Hola crew!

I would like to propose the tag 'lightstep.hostname' is appended to this library. It is present in the Go and JavaScript libraries, and would be useful such that we do not have to append that tag to the tracing constructor manually.

See:

• https://github.com/lightstep/lightstep-tracer-go/blob/eb40dd366f0f992fd08eb681d18c3898fcd68a64/options.go#L41
• https://github.com/lightstep/lightstep-tracer-javascript/blob/e9db0768cf7f3e063b738da479d734844092311e/src/imp/runtime_imp.js#L57

The practical value is to allow us to identify sick nodes for inspection. Whether it is the nodes themselves that are sick, or whether it is some condition about those nodes that is otherwise invisible to the trace (i.e. AZ, connection state or so) -- it's still useful to be able to see an identity to start the inspection.