The value of tmnxOamPingHistorySize indicates the size of the OAM echo
request transmitted for this OAM probe. Zero is returned if no probe
has been transmitted.
When tmnxOamPingCtlTestMode is 'ethCfmLoopback', 'ethCfmTwoWayDelay',
or 'ethCfmTwoWaySlm' the size of the trasmitted layer 2 frame
(including the layer 2 header) is returned.
For other tmnxOamPingCtlTestMode values, the size returned is the
tmnxOamPingCtlSize value.