Introduce `timeseries.instance` to ECS

Beats is introducing a new field for timeseries events. Purpose and meaning is explained here: https://github.com/elastic/beats/pull/10293

`timeseries.instance` field will be attached to events carrying time series: . The objective of it is to allow queries to downsample metrics from multiple time series while consuming them.

Note: this will be introduced in an experimental way, so it may be too soon to add it to ECS, but discussion on the name is useful already.

Start by reading the linked Beats pull request to understand the proposed purpose and meaning of `timeseries.instance`. No ECS file, test, or entry point is named; the issue is currently an experimental naming discussion, so done would require a decided field definition and an agreed ECS change.

Written by the indexing model from the issue text.