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>

tests/plugins/snmp/test_readings.py

@@ -17,10 +17,12 @@ def test_interface_numbers_read_as_a_list():
 def test_interfaces_read_64_bit_counters_except_on_v1():
     v2 = {r.key: r for r in interface_readings([3], "2c")}
     v1 = {r.key: r for r in interface_readings([3], "1")}
-    assert set(v2) == {"if3_status", "if3_in", "if3_out"}
+    assert set(v2) == {"if3_status", "if3_in", "if3_out", "if3_in_errors", "if3_out_errors",
+                       "if3_in_discards", "if3_out_discards"}
     assert v2["if3_in"].oid == "1.3.6.1.2.1.31.1.1.1.6.3" and v2["if3_in"].counter
     assert v1["if3_in"].oid == "1.3.6.1.2.1.2.2.1.10.3"
     assert stored_unit(v2["if3_out"]) == "bits_per_second" and not v2["if3_status"].counter
+    assert v2["if3_in_errors"].oid == "1.3.6.1.2.1.2.2.1.14.3" and stored_unit(v2["if3_in_errors"]) == "per_second"
 
 
 def test_a_custom_line_is_an_oid_a_name_and_a_unit():
@@ -60,6 +62,7 @@ def test_presets_come_in_order_and_names_may_not_repeat():
     ("kilobytes", 2048, "2.0 MiB"),
     ("bits_per_second", 12_345_678, "12.3 Mbit/s"),
     ("bits_per_second", 950, "950 bit/s"),
+    ("bytes_per_second", 3 * 1024 * 1024, "3.0 MiB/s"),
     ("if_status", 2, "down"),
     ("ups_battery", 3, "low"),
     ("if_status", 99, "code 99"),
@@ -140,3 +143,32 @@ def test_a_state_shows_the_device_word_and_falls_back_to_its_level():
     r = Reading("1.1", "raid0_status", "Volume 1 status", "state", states="synology_raid")
     assert reading_text(r, 1.0, {"raid0_status": "Data scrubbing"}) == "Data scrubbing"
     assert reading_text(r, 1.0, {}) == "Busy" and reading_text(r, None, None) == "—"
+
+
+def test_cpu_waiting_on_disk_is_the_wait_counter_s_share_of_the_cpu_s():
+    """#5997: asked for as its five tick counters, kept as one percentage."""
+    linux = {r.key: r for r in device_readings(["linux"], [], "", "2c")}
+    wait = linux["cpu_iowait"]
+    assert wait.oid == "" and stored_unit(wait) == "percent"
+    assert [i.oid for i in wait.inputs] == [f"1.3.6.1.4.1.2021.11.{n}.0" for n in (50, 51, 52, 53, 54)]
+    assert all(i.counter for i in wait.inputs)
+    ticks = {"cpu_ticks_user": 100.0, "cpu_ticks_nice": 0.0, "cpu_ticks_system": 50.0,
+             "cpu_ticks_idle": 800.0, "cpu_ticks_wait": 50.0}
+    assert wait.combine(ticks) == 5.0
+    assert wait.combine(dict.fromkeys(ticks, 0.0)) is None             # no ticks between two polls
+    assert linux["memory_available"].combine(
+        {"memory_free_kb": 1000.0, "memory_buffers_kb": 200.0, "memory_cached_kb": 3000.0}) == 4200.0
+
+
+def test_a_synology_disk_and_volume_read_their_io_and_the_disk_its_reconnects():
+    tables = {t.id: t for t in TABLES["synology"]}
+    io = table_readings(tables["synology_disk_io"], [{"index": "1", "label": "sata1",
+                                                      "columns": ["read", "writes", "load"]}])
+    assert [(r.oid, r.key, r.name, stored_unit(r)) for r in io] == [
+        ("1.3.6.1.4.1.6574.101.1.1.12.1", "storageio1_read", "sata1 read", "bytes_per_second"),
+        ("1.3.6.1.4.1.6574.101.1.1.6.1", "storageio1_writes", "sata1 writes", "per_second"),
+        ("1.3.6.1.4.1.6574.101.1.1.8.1", "storageio1_load", "sata1 load", "percent")]
+    assert tables["synology_disk_io"].keeps("sata2") and not tables["synology_disk_io"].keeps("md2")
+    assert tables["synology_volume_io"].columns[0].oid == "1.3.6.1.4.1.6574.102.1.1.12"
+    disks = {c.key: c.oid for c in tables["synology_disks"].columns}
+    assert disks["retries"] == "1.3.6.1.4.1.6574.2.1.1.8" and disks["identify_fails"] == "1.3.6.1.4.1.6574.2.1.1.10"