Add hw-exporter

[drigz]

This exports a list of connected PCI devices, so that we can track which
GPUs and NICs are installed on the fleet. node-exporter doesn't support
anything quite like this, the ethtool metric doesn't handle NICs that
aren't bound to a kernel driver for example, nor can it identify the
attached GPU.

The new binary is 9MB, and the new process has 21MB RSS, which seems to
mostly be TLS/protobuf dependencies that I guess come in via the
OpenCensus libraries, but could also come in via the PCI library, as
that has the ability to fetch a PCI ID database from the internet if
enabled at runtime (it is not enabled in this binary).

The extra metric load should be offset by
#549.

Example metrics:

# HELP pci_device_count Number of PCI devices by vendor, product, class, and driver.
# TYPE pci_device_count gauge
pci_device_count{class="Bridge",driver="",product="0x7a90",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="",product="0xa700",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="pcieport",product="0x7ab6",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="pcieport",product="Alder Lake-S PCH PCI Express Root Port #13",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="pcieport",product="Alder Lake-S PCH PCI Express Root Port #8",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="pcieport",product="Raptor Lake PCI Express 5.0 Graphics Port (PEG010)",vendor="Intel Corporation"} 1
pci_device_count{class="Bridge",driver="pcieport",product="Raptor Lake PCIe 4.0 Graphics Port",vendor="Intel Corporation"} 1
pci_device_count{class="Communication controller",driver="",product="Alder Lake-S PCH Serial IO UART #0",vendor="Intel Corporation"} 1
pci_device_count{class="Communication controller",driver="mei_me",product="Alder Lake-S PCH HECI Controller #1",vendor="Intel Corporation"} 1
pci_device_count{class="Display controller",driver="nvidia",product="GA104GL [RTX A4000]",vendor="NVIDIA Corporation"} 1
pci_device_count{class="Mass storage controller",driver="ahci",product="Alder Lake-S PCH SATA Controller [AHCI Mode]",vendor="Intel Corporation"} 1
pci_device_count{class="Mass storage controller",driver="nvme",product="IX SN530 NVMe SSD (DRAM-less)",vendor="Sandisk Corp"} 3
pci_device_count{class="Memory controller",driver="",product="Alder Lake-S PCH Shared SRAM",vendor="Intel Corporation"} 1
pci_device_count{class="Multimedia controller",driver="",product="Alder Lake-S HD Audio Controller",vendor="Intel Corporation"} 1
pci_device_count{class="Multimedia controller",driver="",product="GA104 High Definition Audio Controller",vendor="NVIDIA Corporation"} 1
pci_device_count{class="Network controller",driver="atemsys_pci",product="Ethernet Connection (17) I219-LM",vendor="Intel Corporation"} 1
pci_device_count{class="Network controller",driver="igb",product="I210 Gigabit Network Connection",vendor="Intel Corporation"} 1
pci_device_count{class="Network controller",driver="intel-eth-pci",product="0x7aac",vendor="Intel Corporation"} 1
pci_device_count{class="Network controller",driver="intel-eth-pci",product="0x7aad",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH SPI Controller",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH Serial IO I2C Controller #0",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH Serial IO I2C Controller #1",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH Serial IO I2C Controller #2",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH Serial IO I2C Controller #3",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="",product="Alder Lake-S PCH Serial IO SPI Controller #1",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="i801_smbus",product="Alder Lake-S PCH SMBus Controller",vendor="Intel Corporation"} 1
pci_device_count{class="Serial bus controller",driver="xhci_hcd",product="Alder Lake-S PCH USB 3.2 Gen 2x2 XHCI Controller",vendor="Intel Corporation"} 1