Enhancing debug information for JTI

Debug commands supporting Junos telemetry interface (JTI) are enhanced to better support JTI.

 The show network-agent statistics (brief | detail) command output now includes: 

•  Average latency values per sensor, which helps to check the latency of any given sensor on the device. 
• Approximate circular buffer usage per sensor, which provides an early alert if drops are likely for any specific sensor. 
• Time of subscription, which helps to correlate statistics information from the provisioning logs that are taken over a period of multiple subscriptions. The show extension-service request-response clients (brief | detail) command output now includes: 
• The username for which the session was authenticated in a remote procedure call (gRPC) session. If not authenticated, the username field displays as no authentication. This helps to identify which users have requested programmable operations. 
• Login time of the gRPC client, which helps determine how long this client has been active.