I doubt a pre-made commercial product for full-duplex inline bandwidth metering would be priced affordably, and so would never make it to market, though perhaps one of the Fluke or other similar test devices might have a mode in which they could do this.
Any managed switch would be likely to have utilization counters, and if there's not already a display, it would be trivial to hack something together either with SNMP or just on the serial port (an LCD plus a PIC to send "Show int FastEthernet 0/1 | include ..."?)
I strongly recommend getting your hands on an old Cisco FastEthernet managed switch and having a go with some of the various open source SNMP and Netflow tools. You can produce some amazing graphs and displays with minimal configuration effort.
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