Store original message length pre truncation in trace metadata

[RulaKhaled]

We need the length of the array (number of messages).
Not bytes or characters.

[linear]

JS-1247

[RulaKhaled]

We are still debating the “correct” place to store this information. Currently, the SDK does not support metadata at the trace/span level, so we need to determine the best location for it. We’ll revisit this when @cleptric is back

[RulaKhaled]

Until we support _meta field in trace/span level, we are closing this ticket.