ONCE mode supported using gNMI services and JTI

You can include the "ONCE" mode with the Subcribe RPC when subscribing to gRPC Network Management Interface (gNMI) services to export statistics for telemetry monitoring and management using Junos telemetry interface (JTI). ONCE mode ensures that the collector is only streamed telemetry information one time. The Subscribe RPC and subscription parameters are defined in the gnmi.proto file. Streaming telemetry data through gNMI also requires the OpenConfig for Junos OS module.