feat: fan RPM monitoring (#2032)

Adds fan RPM monitoring as a peer to the existing temperature collection, addressing #1918.

---------

Co-authored-by: henrygd <hank@henrygd.me>

agent/fans_test.go

@@ -0,0 +1,87 @@
+//go:build testing
+
+package agent
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// writeFile creates path with parents and writes contents.
+func writeFile(t *testing.T, path, contents string) {
+	t.Helper()
+	require.NoError(t, os.MkdirAll(filepath.Dir(path), 0o755))
+	require.NoError(t, os.WriteFile(path, []byte(contents), 0o644))
+}
+
+// TestReadHwmonFans verifies the /sys/class/hwmon walker:
+//   - picks up fan*_input from every chip,
+//   - keys entries by chip name + sensor label (or fan idx if no label),
+//   - retains 0 RPM for stopped fans,
+//   - tolerates chips with no fan files at all.
+func TestReadHwmonFans(t *testing.T) {
+	root := t.TempDir()
+
+	// hwmon0: Raspberry Pi 5 active cooler — one fan, no label.
+	writeFile(t, filepath.Join(root, "hwmon0", "name"), "pwmfan\n")
+	writeFile(t, filepath.Join(root, "hwmon0", "fan1_input"), "6500\n")
+
+	// hwmon1: a thermal-only chip, no fan files. Must not error.
+	writeFile(t, filepath.Join(root, "hwmon1", "name"), "cpu_thermal\n")
+	writeFile(t, filepath.Join(root, "hwmon1", "temp1_input"), "55000\n")
+
+	// hwmon2: two fans — one stopped (0 RPM) and one labeled "chassis".
+	writeFile(t, filepath.Join(root, "hwmon2", "name"), "nct6798\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan1_input"), "0\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan2_input"), "1200\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan2_label"), "chassis\n")
+
+	fans, err := readHwmonFans(root)
+	require.NoError(t, err)
+
+	assert.Equal(t, map[string]uint16{
+		"pwmfan_fan1":     6500,
+		"nct6798_fan1":    0,
+		"nct6798_chassis": 1200,
+	}, fans)
+}
+
+// TestReadHwmonFansMissingRoot returns an error rather than panicking when the
+// hwmon root doesn't exist (e.g. running on a kernel without hwmon support).
+func TestReadHwmonFansMissingRoot(t *testing.T) {
+	_, err := readHwmonFans(filepath.Join(t.TempDir(), "does-not-exist"))
+	assert.Error(t, err)
+}
+
+// TestReadHwmonFansEmpty returns an empty map (not nil error) when the root
+// exists but contains no chips at all.
+func TestReadHwmonFansEmpty(t *testing.T) {
+	root := t.TempDir()
+	fans, err := readHwmonFans(root)
+	require.NoError(t, err)
+	assert.Empty(t, fans)
+}
+
+func TestFanDiscoveryCache(t *testing.T) {
+	root := t.TempDir()
+	input := filepath.Join(root, "hwmon0", "fan1_input")
+	writeFile(t, filepath.Join(root, "hwmon0", "name"), "chip\n")
+	writeFile(t, input, "1000\n")
+
+	getSensors := newFanSensorCache(root)
+	sensors, err := getSensors()
+	require.NoError(t, err)
+	fans := readFanSensors(sensors)
+	assert.Equal(t, uint16(1000), fans["chip_fan1"])
+
+	writeFile(t, input, "1200\n")
+	writeFile(t, filepath.Join(root, "hwmon0", "fan1_label"), "case\n")
+	sensors, err = getSensors()
+	require.NoError(t, err)
+	fans = readFanSensors(sensors)
+	assert.Equal(t, map[string]uint16{"chip_fan1": 1200}, fans)
+}