netstat: Use procfs parsers for netstat, snmp and snmp6 (#3796)

* netstat: Use procfs parsers for netstat, snmp and snmp6

Replace the hand-written parsers for /proc/net/netstat, /proc/net/snmp
and /proc/net/snmp6 with the parsers from prometheus/procfs
(Proc.Netstat, Proc.Snmp and Proc.Snmp6). The statistics are read via
/proc/self/net, which is equivalent to /proc/net (both are
network-namespace local views of the current process), so the collected
data is unchanged.

Metrics are exposed by iterating the procfs statistics structs, the
same approach the NFS collector uses: fields are *float64 and only the
ones present on the system are exported, matching the previous
behavior. The --collector.netstat.fields flag and the exported metric
names are unchanged; the e2e golden output is identical.

Signed-off-by: neoLsH <43921685+neoLsH@users.noreply.github.com>

* netstat: use explicit package-level metric descriptors

Replace the per-scrape prometheus.NewDesc calls with a pre-built map of
explicit descriptors (netstat_descs_linux.go, generated from the procfs
statistics structs), as requested in review. Collection still reflects over
the procfs structs but looks up the descriptor by "<protocol>_<field>", so
descriptors are allocated once and are source-visible literals for the
AST-based doc generation. Metric names, help strings and types are unchanged.

Signed-off-by: 霏承 <huangleshu.hls@alibaba-inc.com>

* Bump procfs to v0.22.0 to populate legacy TcpExt descriptors

procfs v0.22.0 includes prometheus/procfs#863, which models the 10 legacy
TcpExt counters. With this bump, the explicit descriptors already present in
netstat_descs_linux.go start populating for users whose field regex selects
them, restoring parity with older kernels.

Part of #3796.

Signed-off-by: 霏承 <huangleshu.hls@alibaba-inc.com>

* Add generator and drift test for the netstat descriptor table

netstat_descs_linux.go was previously committed without its generator. Add an
in-repo generator (netstat_descs_gen.go, a build-ignored program run via
GOOS=linux go generate ./collector) that regenerates the explicit descriptor
table from the procfs structs, mirroring the go:generate pattern used elsewhere
in the Prometheus ecosystem.

Add TestNetStatDescsInSync, a drift test that fails if the committed table
diverges from the procfs structs (e.g. after a procfs upgrade) until the table
is regenerated.

Part of #3796.

Signed-off-by: 霏承 <huangleshu.hls@alibaba-inc.com>

---------

Signed-off-by: neoLsH <43921685+neoLsH@users.noreply.github.com>
Signed-off-by: 霏承 <huangleshu.hls@alibaba-inc.com>