🐛 (RUMM-255): Let the Span type use custom

The type defines the icon next to the spans in the dashboard,
and can have the following values:
 - web
 - db
 - cache
 - custom

dd-sdk-android/src/main/kotlin/com/datadog/android/tracing/internal/domain/SpanSerializer.kt

@@ -30,7 +30,7 @@ internal class SpanSerializer(
         jsonObject.addProperty(TAG_DURATION, model.durationNano)
         jsonObject.addProperty(TAG_START_TIMESTAMP, model.startTime + serverOffset)
         jsonObject.addProperty(TAG_ERROR, if (model.isError) 1 else 0)
-        jsonObject.addProperty(TAG_TYPE, "object") // do not know yet what should be here
+        jsonObject.addProperty(TAG_TYPE, "custom")
         addMeta(jsonObject, model)
         addMetrics(jsonObject, model)
         return jsonObject.toString()