Explanation

Entity: LogicalName
Querying the EntityMetadata entity type:
GET [Organization URI]/api/data/v9.0/EntityDefinitions(LogicalName='account') Attribute: LogicalName Retrieving attributes:
GET [Organization URI]/api/data/v9.0/EntityDefinitions(LogicalName='account')/Attributes( Relationship: SchemaName Querying relationship metadata:
Entity relationships can also be queried using the RelationshipDefinitions entity set. You can use a query like the following to get the SchemaName property for every relationship.
GET [Organization URI]/api/data/v9.0/RelationshipDefinitions?$select=SchemaName Global Option Set: Name Querying Global OptionSets:
GET /api/data/v9.0/GlobalOptionSetDefinitions(Name='metric_goaltype')
Note: Retrieving items by name is generally easier because you probably already have some reference to the metadata item name in your code. The following table lists the alternate key properties for retrieving metadata items by name.

Reference:
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/webapi/query-metadata-web-api