hi jacobo, the actual bps rate from the cli will not be in a mib or added (will be available in yang models), but with mrtg or the like you can take the packet counters and have an offline tool compute the rate. I think the following may help for that:
IOS-XR MIB, for multicast traffic is CISCO-IETF-IPMROUTE-MIB
(ftp://ftp.cisco.com/pub/mibs/v2/CISCO-IETF-IPMROUTE-MIB.my).
In this there are "cIpMRoutePkts", "cIpMRouteDifferentInIfPackets", "cIpMRouteOctets" and "cIpMRouteHCOctets"
xander