Fix Zipkin UI failing

[shleshg]

When you are trying to send span to Zipkin without localEndpoint or with serviceName equals to empty string (as opentelemetry-go Zipkin exporter does), Zipkin UI is failing with error, while rendering traces with such error:

TypeError: "e.serviceSummaries is undefined"
    ua TracesTableRow.jsx:140
    React 6
    unstable_runWithPriority scheduler.production.min.js:19
    React 4
    Redux 6
    un traces-action.js:32
react-dom.production.min.js:209:194
    React 5
    unstable_runWithPriority scheduler.production.min.js:19
    React 4
    unstable_runWithPriority scheduler.production.min.js:19
    React 4
    Redux 6
    un traces-action.js:32

To fix this, I add mandatory serviceName parameter to NewExporter function.

Also add assert if serviceName == "" + test for it and fix the example.

[lizthegrey]

You need to follow this step: https://identity.linuxfoundation.org/projects/cncf/individual-signup

[shleshg]

I signed it

[lizthegrey]

it's surprising to me that this is what gofmt does, but the tests pass, so...