Merge branch 'am-clarify-usage-of-data-categories-docs' into 'master'

Optional is default data category

See merge request gitlab-org/gitlab!66613

doc/development/service_ping/metrics_dictionary.md

@@ -37,7 +37,7 @@ Each metric is defined in a separate YAML file consisting of a number of fields:
 | `status`            | yes      | `string`; [status](#metric-statuses) of the metric, may be set to `data_available`, `implemented`, `not_used`, `deprecated`, `removed`, `broken`. |
 | `time_frame`        | yes      | `string`; may be set to a value like `7d`, `28d`, `all`, `none`. |
 | `data_source`       | yes      | `string`; may be set to a value like `database`, `redis`, `redis_hll`, `prometheus`, `system`. |
-| `data_category`     | yes      | `string`; [categories](#data-category) of the metric, may be set to `Operational`, `Optional`, `Subscription`, `Standard`. |
+| `data_category`     | yes      | `string`; [categories](#data-category) of the metric, may be set to `Operational`, `Optional`, `Subscription`, `Standard`. The default value is `Optional`.|
 | `instrumentation_class` | no   | `string`; [the class that implements the metric](metrics_instrumentation.md).  |
 | `distribution`      | yes      | `array`; may be set to one of `ce, ee` or `ee`. The [distribution](https://about.gitlab.com/handbook/marketing/strategic-marketing/tiers/#definitions) where the tracked feature is available.  |
 | `tier`              | yes      | `array`; may be set to one of `free, premium, ultimate`, `premium, ultimate` or `ultimate`. The [tier]( https://about.gitlab.com/handbook/marketing/strategic-marketing/tiers/) where the tracked feature is available. |
@@ -99,7 +99,7 @@ should be changed.
 We use the following categories to classify a metric:
 
 - `Operational`: Required data for operational purposes.
-- `Optional`: Data that is optional to collect. This can be [enabled or disabled](../service_ping/index.md#disable-service-ping) in the Admin Area.
+- `Optional`: Default value for a metric. Data that is optional to collect. This can be [enabled or disabled](../service_ping/index.md#disable-service-ping) in the Admin Area.
 - `Subscription`: Data related to licensing.
 - `Standard`: Standard set of identifiers that are included when collecting data.