Fix ethtool returning 0 for sanitized metrics

[quatre]

The ethtool_linux looks for ethtool stats with their sanitized name which might be different from the name provisioned by ethtool.

This caused node-exporter to return a 0-value for sanitized metrics.

This patch works-around the missing key by copying ethtool stats to their sanitized name.

[SuperQ]

--- FAIL: TestEthToolCollector (0.00s)
    ethtool_linux_test.go:401:  # HELP node_ethtool_align_errors Network interface align_errors
         # TYPE node_ethtool_align_errors untyped
         node_ethtool_align_errors{device="eth0"} 0
         # HELP node_ethtool_info A metric with a constant '1' value labeled by bus_info, device, driver, expansion_rom_version, firmware_version, version.
         # TYPE node_ethtool_info gauge
         node_ethtool_info{bus_info="0000:00:1f.6",device="eth0",driver="e1000e",expansion_rom_version="",firmware_version="0.5-4",version="5.11.0-22-generic"} 1
        +# HELP node_ethtool_port_received_dropped Network interface port_rx_dropped
        +# TYPE node_ethtool_port_received_dropped untyped
        +node_ethtool_port_received_dropped{device="eth0"} 12028
         # HELP node_ethtool_received_broadcast Network interface rx_broadcast
         # TYPE node_ethtool_received_broadcast untyped

[quatre]

Hey! It's not really clear to me why this is not passing. Any idea? Anyway, I'll revisit on Monday!