Erlang interop - logger level and timestamp change

[chulkilee]

Use Erlang logger level

Elixir 1.11 introduces erl_level in metadata to put the correct level, while passing old level for backward compatibility.

Use the "correct" logger level, if found (e.g. in Elixir 1.11)

This will change the level from output json

• Add tests

Use Erlang time

Erlang actually put time metadata - which is unix timestamp with microsecond - so we can get DateTime - while Elixir Logger passes {date, time} tuples which does not have timezone.

Let's use the timestamp from erlang - under the same key (timestamp) for now.

This will change the format of timestamp - adding Z

• Add tests