fix(snmp): a UPS on mains reads "On battery: No, on mains", not "0s" (#5214)

UPS-MIB upsSecondsOnBattery is zero whenever the UPS is on mains power, so
"On battery for 0s" read as a timer that had just started. The reading gets
its own unit, battery_seconds: zero says "No, on mains", anything else
"for <span>". The metric key stays on_battery_for, so stored history keeps.

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

plugins/snmp/readings.py

@@ -69,7 +69,7 @@ PRESETS: dict[str, tuple[str, str, tuple[Reading, ...]]] = {
              Reading("1.3.6.1.2.1.33.1.2.3.0", "runtime_left", "Runtime left", "minutes"),
              Reading("1.3.6.1.2.1.33.1.4.4.1.5.1", "load", "Load", "percent"),
              Reading("1.3.6.1.2.1.33.1.3.3.1.3.1", "input_voltage", "Input", "volts"),
-             Reading("1.3.6.1.2.1.33.1.2.2.0", "on_battery_for", "On battery for", "seconds"))),
+             Reading("1.3.6.1.2.1.33.1.2.2.0", "on_battery_for", "On battery", "battery_seconds"))),
     "linux": ("Linux server", "CPU, memory and load from net-snmp (snmpd's UCD-SNMP-MIB).",
               (Reading("1.3.6.1.4.1.2021.11.11.0", "cpu_used", "CPU used", "percent", scale=-1.0, offset=100.0),
                Reading("1.3.6.1.4.1.2021.10.1.5.1", "load_1m", "Load (1m)", "number", scale=0.01),
@@ -183,6 +183,9 @@ def format_value(unit: str, value: float | None) -> str:
         return human_duration(value / 100)
     if unit == "seconds":
         return human_duration(value)
+    if unit == "battery_seconds":
+        # UPS-MIB upsSecondsOnBattery: zero whenever the UPS is on mains power.
+        return f"for {human_duration(value)}" if value > 0 else "No, on mains"
     if unit == "minutes":
         return human_duration(value * 60)
     if unit == "percent":
@@ -207,6 +210,7 @@ def format_value(unit: str, value: float | None) -> str:
 # What a chart's axis says for a stored unit, and how its values scale for it.
 def chart_axis(unit: str) -> tuple[str, float]:
     return {"centiseconds": ("days", 1 / 8_640_000), "seconds": ("seconds", 1.0),
+            "battery_seconds": ("seconds on battery", 1.0),
             "minutes": ("minutes", 1.0), "percent": ("%", 1.0), "celsius": ("°C", 1.0),
             "volts": ("V", 1.0), "decivolts": ("V", 0.1), "bytes": ("MiB", 1 / 1_048_576),
             "kilobytes": ("MiB", 1 / 1024), "bits_per_second": ("Mbit/s", 1e-6),

tests/plugins/snmp/test_readings.py

@@ -54,6 +54,8 @@ def test_presets_come_in_order_and_names_may_not_repeat():
     ("volts", 121.0, "121 V"),
     ("decivolts", 1203, "120.3 V"),
     ("minutes", 45, "45m"),
+    ("battery_seconds", 0, "No, on mains"),
+    ("battery_seconds", 300, "for 5m"),
     ("kilobytes", 2048, "2.0 MiB"),
     ("bits_per_second", 12_345_678, "12.3 Mbit/s"),
     ("bits_per_second", 950, "950 bit/s"),