Add metadata support for storage sensors

Storage sensors (disk/memory usage) need to fetch two OIDs to calculate
percentages: the 'used' value and the 'size' value. Previously, only the
'used' OID was stored and polled, resulting in meaningless raw values.

Changes:
- Added metadata JSONB field to snmp_sensors table
- Updated Sensor schema to include metadata field
- Updated MikroTik profile to store size_oid in metadata for storage sensors
- Updated PollerWorker to handle percentage calculation sensors:
  - poll_simple_sensor: Standard OID fetch with divisor
  - poll_percentage_sensor: Fetches both used and size OIDs, calculates percentage

Storage sensors now properly calculate and store percentage values during polling.

NOTE: Existing MikroTik devices need to re-run SNMP discovery to populate
the metadata field for storage sensors.

lib/towerops/snmp/poller_worker.ex

@@ -99,40 +99,36 @@ defmodule Towerops.Snmp.PollerWorker do
 
   defp poll_sensors(sensors, client_opts, timestamp) do
     Enum.each(sensors, fn sensor ->
-      case Client.get(client_opts, sensor.sensor_oid) do
-        {:ok, raw_value} ->
-          decoded_value = decode_snmp_value(raw_value)
+      # Check if sensor requires special calculation (e.g., storage percentage)
+      result =
+        if sensor.metadata["calculation"] == "percentage" do
+          poll_percentage_sensor(sensor, client_opts)
+        else
+          poll_simple_sensor(sensor, client_opts)
+        end
 
-          if is_number(decoded_value) do
-            # Calculate actual value using divisor
-            value = decoded_value / sensor.sensor_divisor
+      case result do
+        {:ok, value} ->
+          Snmp.create_sensor_reading(%{
+            sensor_id: sensor.id,
+            value: value,
+            status: "ok",
+            checked_at: timestamp
+          })
 
-            # Determine status based on value (could be enhanced with thresholds)
-            status = "ok"
+        {:error, :non_numeric} ->
+          Logger.debug("Non-numeric SNMP value for sensor #{sensor.sensor_descr}")
 
-            # Save the reading
-            Snmp.create_sensor_reading(%{
-              sensor_id: sensor.id,
-              value: value,
-              status: status,
-              checked_at: timestamp
-            })
-          else
-            Logger.debug("Non-numeric SNMP value for sensor #{sensor.sensor_descr}")
-
-            # Save reading with nil value
-            Snmp.create_sensor_reading(%{
-              sensor_id: sensor.id,
-              value: nil,
-              status: "unknown",
-              checked_at: timestamp
-            })
-          end
+          Snmp.create_sensor_reading(%{
+            sensor_id: sensor.id,
+            value: nil,
+            status: "unknown",
+            checked_at: timestamp
+          })
 
         {:error, reason} ->
           Logger.warning("Failed to poll sensor #{sensor.sensor_descr}: #{inspect(reason)}")
 
-          # Save error status
           Snmp.create_sensor_reading(%{
             sensor_id: sensor.id,
             value: nil,
@@ -143,6 +139,43 @@ defmodule Towerops.Snmp.PollerWorker do
     end)
   end
 
+  defp poll_simple_sensor(sensor, client_opts) do
+    case Client.get(client_opts, sensor.sensor_oid) do
+      {:ok, raw_value} ->
+        decoded_value = decode_snmp_value(raw_value)
+
+        if is_number(decoded_value) do
+          # Calculate actual value using divisor
+          value = decoded_value / sensor.sensor_divisor
+          {:ok, value}
+        else
+          {:error, :non_numeric}
+        end
+
+      {:error, reason} ->
+        {:error, reason}
+    end
+  end
+
+  defp poll_percentage_sensor(sensor, client_opts) do
+    # For percentage sensors (like storage), fetch both used and size
+    size_oid = sensor.metadata["size_oid"]
+    used_oid = sensor.sensor_oid
+
+    with {:ok, used_raw} <- Client.get(client_opts, used_oid),
+         {:ok, size_raw} <- Client.get(client_opts, size_oid),
+         used when is_number(used) <- decode_snmp_value(used_raw),
+         size when is_number(size) <- decode_snmp_value(size_raw),
+         true <- size > 0 do
+      percentage = used / size * 100
+      {:ok, percentage}
+    else
+      {:error, reason} -> {:error, reason}
+      false -> {:error, :division_by_zero}
+      _ -> {:error, :non_numeric}
+    end
+  end
+
   defp poll_interfaces(interfaces, client_opts, timestamp) do
     Enum.each(interfaces, fn interface ->
       # Poll interface stats: ifInOctets, ifOutOctets, ifInErrors, ifOutErrors

lib/towerops/snmp/profiles/mikrotik.ex

@@ -235,7 +235,11 @@ defmodule Towerops.Snmp.Profiles.Mikrotik do
             sensor_unit: "%",
             sensor_divisor: 1,
             last_value: percent,
-            status: storage_status(type, percent)
+            status: storage_status(type, percent),
+            metadata: %{
+              size_oid: size_oid,
+              calculation: "percentage"
+            }
           }
         end
       end)

lib/towerops/snmp/sensor.ex

@@ -16,6 +16,7 @@ defmodule Towerops.Snmp.Sensor do
     field :monitored, :boolean, default: true
     field :last_value, :float
     field :last_checked_at, :utc_datetime
+    field :metadata, :map, default: %{}
 
     belongs_to :snmp_device, Towerops.Snmp.Device
 
@@ -37,7 +38,8 @@ defmodule Towerops.Snmp.Sensor do
       :sensor_divisor,
       :monitored,
       :last_value,
-      :last_checked_at
+      :last_checked_at,
+      :metadata
     ])
     |> validate_required([:snmp_device_id, :sensor_type, :sensor_index, :sensor_oid])
     |> unique_constraint([:snmp_device_id, :sensor_index])

priv/repo/migrations/20260104190824_add_sensor_metadata.exs

@@ -0,0 +1,9 @@
+defmodule Towerops.Repo.Migrations.AddSensorMetadata do
+  use Ecto.Migration
+
+  def change do
+    alter table(:snmp_sensors) do
+      add :metadata, :map, default: %{}
+    end
+  end
+end