Get interface counter in gNMI

[ntt-omw]

There is no description of Interface Counter in the Supported attribute of the OpenConfig YANG tree in the HLD of OpenConfig_Interfaces.md.
However, it is confirmed that Interface Counter can be obtained via gNMI.
I will add the Interface Counter to OpenConfig_Interfaces.md

https://github.com/sonic-net/SONiC/blob/master/doc/mgmt/OpenConfig_Interfaces.md

[nakano-omw]

The PR is here #1794