Add labels and deprecate tags on Intake API

[simitt]

Follow up from #1826:

Change Intake API to allow for sending labels rather than tags:

Option (1):
deprecate current: { transaction: context: { tags: {}}}
introduce: { transaction: labels: {}, context: {}}

Option (2):
deprecate current: { transaction: context: { tags: {}}}
introduce: { transaction: context: { labels: {}}}

Option (3) - keep what we have:
{ transaction: context: { tags: {}}}

The format would be the same for errors and spans.
@elastic/apm-agent-devs please vote on what you'd prefer

[beniwohli]

I vote for Option 1, tags in the context always felt a bit awkward

[felixbarny]

I also vote on 1 as we are planning to remove context from the intake api anyway now that there is no context in ES documents as of 7.0 (#411).

[graphaelli]

closing for now, will reopen if we decide to tackle this for 8.0

[simitt]

When we implement this we should probably deprecate tags before removing them completely (BC), to ensure a smooth upgrade path with the agents.