[mqtt] set the proper unit in the state description (#13604)

So that other pieces of openhab can know what unit it's going to be,
without it having a value yet. Importantly, any necessary conversion
that need to be applied to the other portion of the state description -
min, max, and step.

See also https://github.com/openhab/openhab-core/pull/3132

Signed-off-by: Cody Cutrer <cody@cutrer.us>

bundles/org.openhab.binding.mqtt.generic/src/main/java/org/openhab/binding/mqtt/generic/values/NumberValue.java

@@ -149,6 +149,11 @@ public class NumberValue extends Value {
         if (min != null) {
             builder = builder.withMinimum(min);
         }
-        return builder.withStep(step).withPattern("%s %unit%");
+        if (!unit.equals(Units.ONE)) {
+            builder.withPattern("%s " + unit);
+        } else {
+            builder.withPattern("%s %unit%");
+        }
+        return builder.withStep(step);
     }
 }