Support array of strings as a value for tags/labels

## Description of the issue
Currently tags/labels are stored as key/value pairs in ES, however, we have seen users expecting tags to be stored as an array of strings, this is consistent with [ECS-style tags](https://www.elastic.co/guide/en/ecs/current/ecs-base.html).

I'm proposing to support array of strings for adding tags/labels both on the APM server as well as the agent APIs.

The alternative to this approach is to have both labels for key/values and tags for arrays but since supporting just labels (with support for arrays) is more generic, IMO, we should implement the proposed solution.

An example for JavaScript:
```js
apm.addTags({'teams': ['team1','team2']})
```

## What we are voting on
Is this an acceptable solution for agents/apm server?
cc @elastic/apm-agent-devs , @elastic/apm-server

## Vote

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| RUM |

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| APM Server |

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No source files, tests, or entry points are named. Start by reviewing the agent tag/label APIs and the APM server's Elasticsearch mapping and intake handling, then compare them with the ECS-style tags example. Done means the teams agree on the array representation, affected APIs and compatibility behavior, with implementation scope identified.

Written by the indexing model from the issue text.