Allowing bool and numbers for `labels`

[simitt]

The current description of labels states that the values need to be keywords:

Key/value pairs. Can be used to add meta information to events. Should not contain nested objects. All values are stored as keyword.

Can we extend this to also allow at least boolean and number values? APM uses labels to allow setting user defined information about events. We already allow keywords, numbers and booleans. Numbers could be used for aggregations on these fields. (see elastic/apm-server#828 for more background).

[ebeahan]

I've opened an RFC to start a discussion around this topic: #1341

Let's capture any new thoughts or feedback in that conversation.