Update Usage Ping guide

  - Separate feature flag is not required anymore
  - One feature flag can be used for multiple events
  - Use feature flag defined in know_events

  Related with https://gitlab.com/gitlab-org/gitlab/-/issues/276948

doc/development/usage_ping/index.md

@@ -497,6 +497,7 @@ Implemented using Redis methods [PFADD](https://redis.io/commands/pfadd) and [PF
      redis_slot: compliance
      expiry: 42  # 6 weeks
      aggregation: weekly
+     feature_flag: usage_data_i_compliance_credential_inventory
    ```
 
    Keys:
@@ -528,7 +529,7 @@ Implemented using Redis methods [PFADD](https://redis.io/commands/pfadd) and [PF
      aggregation.
    - `aggregation`: may be set to a `:daily` or `:weekly` key. Defines how counting data is stored in Redis.
      Aggregation on a `daily` basis does not pull more fine grained data.
-   - `feature_flag`: optional `default_enabled: :yaml`. If no feature flag is set then the tracking is enabled. For details, see our [GitLab internal Feature flags](../feature_flags/index.md) documentation. The feature flags are owned by the group adding the event tracking.
+   - `feature_flag`: optional `default_enabled: :yaml`. If no feature flag is set then the tracking is enabled. One feature flag can be used for multiple events. For details, see our [GitLab internal Feature flags](../feature_flags/index.md) documentation. The feature flags are owned by the group adding the event tracking.
 
 Use one of the following methods to track events:
 
@@ -567,8 +568,6 @@ Use one of the following methods to track events:
 
 1. Track event in API using `increment_unique_values(event_name, values)` helper method.
 
-   To be able to track the event, Usage Ping must be enabled and the event feature `usage_data_<event_name>` must be enabled.
-
    Arguments:
 
    - `event_name`: event name.
@@ -612,10 +611,6 @@ Use one of the following methods to track events:
 
    API requests are protected by checking for a valid CSRF token.
 
-   To increment the values, the related feature `usage_data_<event_name>` should be
-   set to `default_enabled: true`. For more information, see
-   [Feature flags in development of GitLab](../feature_flags/index.md).
-
    ```plaintext
    POST /usage_data/increment_unique_users
    ```
@@ -640,8 +635,6 @@ Use one of the following methods to track events:
    Usage Data API is behind  `usage_data_api` feature flag which, as of GitLab 13.7, is
    now set to `default_enabled: true`.
 
-   Each event tracked using Usage Data API is behind a feature flag `usage_data_#{event_name}` which should be `default_enabled: true`
-
    ```javascript
    import api from '~/api';