metrics: use Linux 6.2+ stat fast path for CurrentFDs

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As of Linux 6.2 (torvalds/linux@f1f1f25699),
the stat size of the /proc/self/fd directory is the number of open file
descriptors, so CurrentFDs no longer needs to walk the directory on
modern kernels. The walk is O(n) in the number of open file descriptors,
which adds up on servers like derper that hold many connections and
report this metric on every scrape: at 10k open fds, the walk takes
about 2.2ms while an fstat takes a constant 300ns.

We fstat a directory file descriptor held open for the process lifetime
rather than calling stat with the path, because syscall.Stat copies the
path string to a NUL-terminated buffer on every call and the metric
must remain allocation-free. Kernels older than 6.2 report a size of
zero, in which case we fall back to the directory walk.

Updates #2784

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I1742db2ae1938c77f9b7d137b167ab4a2b6b6167

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