Add documentation for metrics definition

doc/development/usage_ping/metrics_dictionary.md

+---
+stage: Growth
+group: Product Intelligence
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Metrics Dictionary Guide
+
+This guide describes Metrics Dictionary and how it's implemented
+
+## Metrics Definition and validation
+
+We are using [JSON Schema](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/schema.json) to validate the metrics definition.
+
+This process is meant to ensure consistent and valid metrics defined for Usage Ping. All metrics *must*:
+
+- Comply with the definied [JSON schema](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/schema.json).
+- Have a unique `full_path` .
+- Have an owner.
+
+All metrics are stored in YAML files:
+
+- [`config/metrics`](https://gitlab.com/gitlab-org/gitlab/-/tree/master/config/metrics)
+
+Each metric is definied in a separate YAML file consisting of a number of fields:
+
+| Field               | Required | Additional information                                         |
+|---------------------|----------|----------------------------------------------------------------|
+| `name`              | yes      |                                                                |
+| `description`       | yes      |                                                                |
+| `value_type`        | yes      |                                                                |
+| `status`            | yes      |                                                                |
+| `default_generation`| yes      | Default generation path of the metric. One full_path value. (1) |
+| `full_path`         | yes      | Full path of the metric for one or multiple generations. Path of the metric in Usage Ping payload. (1) |
+| `group`             | yes      | The [group](https://about.gitlab.com/handbook/product/categories/#devops-stages) that owns the metric. |
+| `time_frame`        | yes      | `string`; may be set to a value like "7d"                             |
+| `data_source`       | yes      | `string`: may be set to a value like `database` or `redis_hll`.       |
+| `distribution`      | yes      | The [distribution](https://about.gitlab.com/handbook/marketing/strategic-marketing/tiers/#definitions) where the metric applies. |
+| `tier`              | yes      | The [tier]( https://about.gitlab.com/handbook/marketing/strategic-marketing/tiers/) where the metric applies. |
+| `product_category`  | no       | The [product category](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/categories.yml) for the metric. |
+| `stage`             | no       | The [stage](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/stages.yml) for the metric. |
+| `milestone`         | no       | The milestone when the metric is introduced. |
+| `milestone_removed` | no       | The milestone when the metric is removed. |
+| `introduced_by_url` | no       | The URL to the Merge Request that introduced the metric. |
+
+1. The default generation path is the location of the metric in the Usage Ping payload.
+   The `full_path` is the list locations for multiple Usage Ping generaations.
+
+### Example metric definition
+
+The linked [`uuid`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/license/uuid.yml)
+YAML file includes an example metric definition, where the `uuid` metric is the GitLab
+instance unique identifier.
+
+```yaml
+name: uuid
+description: GitLab instance unique identifier
+value_type: string
+product_category: collection
+stage: growth
+status: data_available
+default_generation: generation_1
+full_path:
+  generation_1: uuid
+  generation_2: license.uuid
+milestone: 9.1
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/1521
+group: group::product intelligence
+time_frame: none
+data_source: database
+distribution: [ee, ce]
+tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']
+```