Calculate and add field `gl2_ingested_message_size` to display number of bytes in `message`field

In contrast to `gl2_accounted_message_size` which stores the number of bytes of all fields stored in Elasticsearch, there should be a field, storing the number of bytes of **solely the message field**.

We as a security services provider charge our customers for the size of logs ingested, and we currently have huge problems in using Graylog because we have no metrics available to see how much data the customer ingested into Graylog.

### Current behaviour
No metrics to see the raw data size of a log
### Expected behaviour
For each log, calculate a field `gl2_ingested_message_size` that contains the size of the `message` field in bytes.