#1733 telemetry: the Monitor's PCIe tile had no link for AMD cards on Linux

Description

_Nvml.read() reports pcie_gen, pcie_gen_max, pcie_width and the two
throughput counters; _Amd.read() (the amdgpu sysfs reader from #301)
reports none of them, so on Linux the Monitor's PCIe tile shows only
dashes while the NVIDIA tile fills in.

This adds the link the card actually gets: the narrowest and slowest hop
between the root port and the card, from each hop's max_link_speed and
max_link_width.  A card that negotiates Gen4 on its own hop but sits
behind a Gen3 root port really runs at Gen3, and that is the number a
bandwidth budget needs.  pcie_own_gen keeps the card's own hop aside so
the gap is visible, and pcie_path reports how many hops could be read --
the bottleneck is only claimed when all of them could, because a kernel
or a container that hides part of /sys/devices would otherwise report the
card's own hop as the whole path.

Measured on one MI50 32 GB in a Gen3 x16 slot (lspci: the root port at
00:02.0 runs at 8 GT/s x16, the bridge above the card is downgraded, and
the card's own hop negotiates 16 GT/s), /metrics reports pcie_gen 3,
pcie_gen_max 3, pcie_own_gen 4, pcie_width 16 and pcie_path 4/4.

Throughput is deliberately left out: amdgpu's pcie_bw is three counters
whose documented use is a total, not a direction, while _Nvml's
pcie_rx_mb is receive-only, so filling that name from pcie_bw would
label a combined figure as one direction.  Happy to follow up separately
if you want the tile to show a rate.

Hardware the numbers come from: one Radeon Instinct MI50 32 GB (gfx906,
PCI 1002:66a1, VBIOS 113-D1631711-100) on a Xeon E5-2696 v4 (Broadwell,
no AVX-512), in a PCIe 3.0 x16 slot, 128 GB RAM, Ubuntu, kernel
7.0.0-34-generic, ROCm 7.14 in a container.