mountstats: add --collector.mountstats.nfs.aggregate-transports flag

[evkuzin]

Adds an opt-in flag, --collector.mountstats.nfs.aggregate-transports, that sums the per-transport NFS metrics across all export entries of a mount and emits a single series per mount (no transport label).

Before 29f6ec0a, mountstats_linux only read the first export entry from /proc/self/mountstats, which silently underreported every transport metric on mounts using NFS nconnect. That commit correctly fixed the numbers by iterating all transports and adding a transport index label.

The fix is right, but for hosts with many NFS mounts and high nconnect values it multiplies the existing (already large) node_mountstats_nfs_* cardinality by nconnect. On a host with tens of mounts, enabling nconect will turns into tens of thousands of additional series per scrape (multiplies all nfs metrics by nconnect value) which is enough to be a real operational problem for the monitoring.

When the new flag is set:

The 10 node_mountstats_nfs_transport_* descriptors are built without the transport label, restoring the pre-29f6ec0a schema and cardinality.
All export entries are aggregated into a single value per mount before being emitted:

• Counters (Bind, Connect, Sends, Receives, BadTransactionIDs, CumulativeBacklog, CumulativeSendingQueue, CumulativePendingQueue) → sum — matches the kernel's actual mount-wide totals.
• IdleTimeSeconds (gauge) → min — represents "time since the mount last saw any RPC traffic on any transport".
• MaximumRPCSlotsUsed (gauge) → max — the busiest transport's peak.

Default is false, so existing scrapes are unchanged.

Happy to add some tests as well.

[evkuzin]

@SuperQ could you take a look when you have time please?