feat(rum): deprecate addTags in favor of addLabels

[vigneshshanmugam]

• first step of Rename addTags to addLabels #215
• Opentracing addTags still works and under the hood calls span.addLabels for adding labels.

[eliaskardel]

I am running the latest version of all components elastic search, Kibana, APM Server, RUM JS agent.

The addLabel() function doesnt give me anything on the other side. I switched on debug in the agent and saw my labels in the developer console. However they seem to not end up in eleastic search.

[eliaskardel]

            const transaction = apm.startTransaction('transaction1', 'custom')
            const httpSpan = transaction.startSpan('span2', 'custom')
            httpSpan.addLabels({'test_label' : 'test_label'})
            httpSpan.end()
            transaction.end()