You are looking to get API that can get bgppeer2peer state table.
The BGP4mib has a bgpPeerState oid that can be used for this.
A simple get of the following will pull peer state of a model. you get the same result pulling it from the device or the app model. We see 2 peers with a value 6 corresponding to established.
/spectrum/restful/model/0x#####?attr=0x213650
The documented approach is Condition-Based BGP Peer Session Monitoring