Make type and len class attributes of OFPInstructionMeter

Make type and len class attributes of OFPInstructionMeter
and update JSON accordingly.

This is in keeping with the implementation of other OFPInstruction classes.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>