feat(target): expose gRPC state metric

[senthilsam]

Introduces gnmic_target_grpc_status_code metric to reflect grpc status code at runtime

# HELP gnmic_target_grpc_status_code The last gRPC status code received from the target. Default is 2 (Unknown) until the first response is received. See https://pkg.go.dev/google.golang.org/grpc/codes. Values: 0=OK, 1=Canceled, 2=Unknown, 3=InvalidArgument, 4=DeadlineExceeded, 5=NotFound, 6=AlreadyExists, 7=PermissionDenied, 8=ResourceExhausted, 9=FailedPrecondition, 10=Aborted, 11=OutOfRange, 12=Unimplemented, 13=Internal, 14=Unavailable, 15=DataLoss, 16=Unauthenticated.
# TYPE gnmic_target_grpc_status_code gauge
gnmic_target_grpc_status_code{name="test-sim-amps-device-1"} 0

During authentication error:

gnmic log
time=2026-07-22T09:33:39.375+05:30 level=INFO msg="subscription receive error" target=test.device.net subscription=test-sample-5m err="rpc error: code = Unauthenticated desc = Authentication has failed"

Generated metric

# HELP gnmic_target_grpc_status_code The last gRPC status code received from the target. Default is 2 (Unknown) until the first response is received. See https://pkg.go.dev/google.golang.org/grpc/codes. Values: 0=OK, 1=Canceled, 2=Unknown, 3=InvalidArgument, 4=DeadlineExceeded, 5=NotFound, 6=AlreadyExists, 7=PermissionDenied, 8=ResourceExhausted, 9=FailedPrecondition, 10=Aborted, 11=OutOfRange, 12=Unimplemented, 13=Internal, 14=Unavailable, 15=DataLoss, 16=Unauthenticated.
# TYPE gnmic_target_grpc_status_code gauge
gnmic_target_grpc_status_code{name="test.device.net"} 16