feat(snmp): a NAS's disk and volume I/O, CPU waiting on disk, available memory, interface errors (#5997)

- Synology: what each disk and volume reads and writes, bytes and requests
  a second and the load DSM gives it (SYNOLOGY-STORAGEIO- and -SPACEIO-MIB,
  64-bit byte counters), and each disk's reconnects and sectors not found.
- Linux server: CPU waiting on disk, the wait tick counter's share of the
  CPU's, and memory available (free + buffers + cache). A reading may now be
  worked out from others: the poll asks for its inputs, rates each counter,
  and stores only the result.
- Interface traffic: errors and discards in and out, per second.
- byte_count unit: a byte counter shown as bytes a second.

Asustor's MIB has nothing more for this; its Linux server readings gain the
same CPU wait and available memory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/snmp/scheduler.py

@@ -139,7 +139,7 @@ async def _do_poll(app: "Quart", poll=None, walk=None) -> None:
                 scans[d.id] = await walk(d.address, d.port, creds, tables_for(d.presets or []))
             found = scans.get(d.id)
             asked[d.id] = readings_for(d, found.rows if found is not None and found.error is None else None)
-            return await poll(d.address, d.port, creds, [(r.oid, r.key) for r in asked[d.id]])
+            return await poll(d.address, d.port, creds, [(r.oid, r.key) for r in _asks(asked[d.id])])
 
         results = await asyncio.gather(*(one(d) for d in devices), return_exceptions=True)
 
@@ -158,23 +158,35 @@ async def _do_poll(app: "Quart", poll=None, walk=None) -> None:
                 device.tables, device.tables_at = p.scan.rows, now
             device.last_error = result.error
             missing = set(result.missing)
-            for reading in asked[device.id]:
+
+            def read(reading):
                 value = result.readings.get(reading.key)
                 if reading.unit == "state":
                     # Its word is shown as answered; its level is stored,
                     # unless Steward doesn't know the word (level None).
                     raw = value if value is not None else result.texts.get(reading.key)
                     if raw is None:
-                        continue
+                        return None
                     p.texts[reading.key], value = state_of(reading, raw)
-                elif value is None:
+                    return value
+                if value is None:
                     if reading.key in result.texts:      # a word where a number was asked for
                         missing.add(reading.key)
-                    continue
-                elif reading.counter:
-                    value = RATES.rate(device.id, reading, value, now)
+                    return None
+                if reading.counter:
+                    return RATES.rate(device.id, reading, value, now)
+                return value * reading.scale + reading.offset
+
+            for reading in asked[device.id]:
+                if reading.inputs:
+                    # Every input is read, so each counter keeps its last sample.
+                    got = {i.key: read(i) for i in reading.inputs}
+                    if any(i.key in missing for i in reading.inputs):
+                        missing -= {i.key for i in reading.inputs}
+                        missing.add(reading.key)
+                    value = None if None in got.values() else reading.combine(got)
                 else:
-                    value = value * reading.scale + reading.offset
+                    value = read(reading)
                 if value is not None:
                     p.values.append((reading.key, value))
             p.missing = sorted(missing)
@@ -183,6 +195,12 @@ async def _do_poll(app: "Quart", poll=None, walk=None) -> None:
     await held.flush(STORE_WAIT_S)
 
 
+def _asks(readings: list) -> list:
+    """What the device is asked for: each reading, or the inputs of one
+    worked out from others."""
+    return [i for r in readings for i in (r.inputs or (r,))]
+
+
 async def _store(app: "Quart", rounds: list[list[Polled]]) -> None:
     """Write held polls, oldest first, in one transaction."""
     from steward.core.alerts import MetricBatch