Add docs in metrics dictionary for object value type

We should document when the object type can be uses for a metric definition

For object type we should require a JSON schema and a data validation agains the linked JSON schema

Existing object types

• topology*
• usage_activity_by_stage.monitor.projects_with_enabled_alert_integrations_histogram )

When considering adding an objects type for metric in Usage Ping, consider answering to following questions

• Does data make sense all together only?
• Does data belong to same product category, group, stage?
• is data for the same time interval?