Allow adding arbitrary tags on span

[maximede]

I'd like to be able to add tags on a span, at runtime.

Similar to what brave does https://github.com/openzipkin/brave/blob/master/brave/README.md#customizing-spans

span.tag("clnt/finagle.version", "6.36.0");

[jcchavezs]

As per our chat @maximede:

The main idea is to support default tags on the client/server instrumentation. Similar to what it is on https://github.com/openzipkin/zipkin-js/blob/e68cc61/packages/zipkin/src/tracer/index.js#L40 but for httpClient and httpServer.

Implementation should be as easy as adding another parameter to https://github.com/openzipkin/zipkin-js/blob/e68cc61/packages/zipkin/src/instrumentation/httpClient.js#L9 declaring the tags.